Method of making zirconia-containing nanoparticles

ABSTRACT

A method of preparing zirconia-containing nanoparticles and a method of preparing a composite material that includes the zirconia-containing nanoparticles are provided. A method of treating a zirconium carboxylate salt solution to remove alkali metal ions and alkaline earth ions is provided. The treated solution can be used as a feedstock to prepare the zirconia-containing nanoparticles. Additionally, a continuous hydrothermal reactor system is provided that can be used, for example, to prepare the zirconia-containing nanoparticles.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.12/810,642, filed Jun. 25, 2010, now U.S Pat. No. 8,647,510, which is anational stage filing under 35 U.S.C. 371 of PCT/US2008/087385, filedDec. 18, 2008, expired, which claims priority to U.S. ProvisionalApplication No. 61/017,326, filed Dec. 28, 2007, the disclosures ofwhich are incorporated by reference in their entireties herein.

BACKGROUND

Zirconia particles can be added to an organic matrix such as a polymericmaterial to increase the index of refraction or x-ray opacity of theorganic matrix while retaining the optical transmission of the organicmatrix. The extent to which the x-ray opacity and/or refractive index ofthe organic matrix can be increased is dependent on the percent loadingof zirconia in the organic matrix and on characteristics of the zirconiaparticles such as the percent crystallinity, the crystalline structure,the primary particle size, and the degree of association between primaryparticles.

Crystalline zirconia usually has a higher refractive index and a greaterx-ray scattering capability than amorphous zirconium-containingmaterials. The optical transmission is often a function of the size ofthe zirconia particles. As the primary particle size increases and/orthe degree of association between primary particles increases, theoptical transmission can be reduced. The percent loading limit ofzirconia particles in an organic matrix material is usually a functionof both the extent of particle association and the particle aspectratio. As the extent of particle association increases, the percentloading limit of the zirconia particles in an organic matrix tends todecrease. Similarly, as the aspect ratio of the zirconia particlesincreases, the percent loading limit of the zirconia particles in anorganic matrix tends to decrease.

SUMMARY

A method of preparing zirconia-containing nanoparticles and a method ofpreparing a composite material that includes the zirconia-containingnanoparticles are provided. A method of treating a solution containing adissolved zirconium carboxylate salt is provided. The method of treatingremoves alkali metal ion and alkaline earth ion impurities from thesolution. The treated solution can be used as a feedstock or as part ofthe feedstock to prepare the zirconia-containing nanoparticles.Additionally, a continuous hydrothermal reactor system is provided thatcan be used, for example, to prepare the zirconia-containingnanoparticles.

In a first aspect, a method of preparing zirconia-containingnanoparticles is provided. The method includes preparing a feedstockthat contains a dissolved zirconium carboxylate salt, wherein thecarboxylate or an acid thereof has no greater than 4 carbon atoms. Thefeedstock has greater than 5 weight percent solids. The method furtherincludes subjecting the feedstock to a single hydrothermal treatment inwhich at least 90 weight percent of a total amount of the zirconium inthe feedstock is converted to zirconia-containing nanoparticles that arecrystalline and non-associated.

In a second aspect, a method of preparing a composite material isprovided. The method includes preparing zirconia-containingnanoparticles as described above that are crystalline andnon-associated. The method further includes suspending or dispersing thezirconia-containing nanoparticles in an organic matrix.

In a third aspect, a method of treating a solution of a zirconiumcarboxylate salt is provided. The method includes contacting thesolution with a cation exchange resin in a hydrogen form. The methodfurther includes sorbing at least 50 mole percent of an alkali metalion, an alkaline earth ion, or a mixture thereof from the solution ontothe cation exchange resin.

In a fourth aspect, a continuous hydrothermal reactor system isprovided. The continuous hydrothermal reactor system includes a tubularreactor having an interior surface that contains a fluorinated polymericmaterial.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1 is a schematic of an exemplary continuous hydrothermal reactorsystem.

FIG. 2 shows the volume distribution of one example and one comparativeexample of zirconia-containing nanoparticles.

FIG. 3 shows an x-ray diffraction scan for exemplary zirconia-containingnanoparticles.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention.

DETAILED DESCRIPTION

Methods of making zirconia nanoparticles and methods of making compositematerials that contain the zirconia-containing nanoparticles aredescribed. A feedstock containing a dissolved zirconium carboxylate saltis subjected to a single hydrothermal treatment. The resultingzirconia-containing nanoparticles are typically crystalline andnon-associated. The zirconia-containing nanoparticles can be dispersedor suspended in an organic matrix to provide transparent or translucentcomposite materials having a high index of refraction, a high x-rayopacity, or a combination thereof.

As used herein, the terms “a”, “an”, and “the” are used interchangeablywith “at least one” to mean one or more of the elements being described.

As used herein, the term “associated” refers to a grouping of two ormore primary particles that are aggregated and/or agglomerated.Similarly, the term “non-associated” refers to two or more primaryparticles that are free or substantially free from aggregation and/oragglomeration.

As used herein, the term “aggregation” refers to a strong association oftwo or more primary particles. For example, the primary particles may bechemically bound to one another. The breakdown of aggregates intosmaller particles (e.g., primary particles) is generally difficult toachieve.

As used herein, the term “agglomeration” refers to a weak association oftwo or more primary particles. For example, the primary particles may beheld together by charge or polarity. The breakdown of agglomerates intosmaller particles (e.g., primary particles) is less difficult than thebreakdown of aggregates into smaller particles.

As used herein, the term “primary particle size” refers to the size of anon-associated single crystal zirconia particle. X-ray Diffraction (XRD)is typically used to measure the primary particle size using the methoddescribed herein.

As used herein, the term “hydrothermal” refers to a method of heating anaqueous medium to a temperature above the normal boiling point of theaqueous medium at a pressure that is equal to or greater than thepressure required to prevent boiling of the aqueous medium.

As used herein, the term “zirconia” refers to various stoichiometricformulas for zirconium oxides. The most typical stoichiometric formulais ZrO₂, which is also known as zirconium oxide and zirconium dioxide.The zirconia may contain up to 30 weight percent, up to 25 weightpercent, up to 20 weight percent, up to 15 weight percent, up to 10weight percent, or up to 5 weight percent of other chemical moietiessuch as, for example, yttrium oxide or various organic materials sorbedon the surface.

As used herein, the term “organic matrix” refers to a polymeric materialor to a precursor of a polymeric material such as a monomer or oligomer.Stated differently, the organic matrix can be a polymerizable material,a polymerized material, or a mixture thereof.

As used herein, the term “in the range of” includes the endpoints of therange. For example, in the range of 1 to 10 includes the numbers 1, 10,and all numbers between 1 and 10.

In a first aspect, a method of preparing zirconia-containingnanoparticles is provided. Hydrothermal technology is used to preparethe zirconia-containing nanoparticles from a feedstock that contains adissolved zirconium carboxylate salt. The zirconia-containingnanoparticles are typically crystalline and non-associated.

The carboxylate present in the feedstock has no greater than 4 carbonatoms. Suitable carboxylates include formate, acetate, propionate (i.e.,n-propionate), butyrate (i.e., n-butyrate, iso-butyrate, or a mixturethereof), or a combination thereof. Typically, some of the correspondingcarboxylic acids of these carboxylates can also be present in thefeedstock. That is, the carboxylate and/or an acid thereof in thefeedstock has no greater than 4 carbon atoms. As used herein, the phrase“carboxylate and/or acid thereof” means the carboxylate, thecorresponding acid of the carboxylate, or a combination thereof.

The feedstock is usually free or substantially free of any carboxylateand/or acid thereof that has greater than 4 carbon atoms. As usedherein, the term “substantially free” with reference to a carboxylateand/or acid thereof that has greater than 4 carbon atoms typically meansthat these materials are not added intentionally to the feedstock butmay be present as an impurity in another component of the feedstock suchas in the zirconium carboxylate salt. There is typically less 1 molepercent, less than 0.5 mole percent, less than 0.3 mole percent, lessthan 0.2 mole percent, less than 0.1 mole percent, less than 0.05 molepercent, less than 0.02 mole percent, or less than 0.01 mole percent ofthese materials present based on the total carboxylates and/or acidsthereof in the feedstock. That is, the amount of carboxylate and/or acidthereof having greater than 4 carbon atoms in the feedstock is in therange of 0 to 1 mole percent, in the range of 0 to 0.5 mole percent, inthe range of 0 to 0.3 mole percent, in the range of 0 to 0.2 molepercent, in the range of 0 to 0.1 mole percent, in the range of 0 to0.05 mole percent, in the range of 0 to 0.02 mole percent, or in therange of 0 to 0.01 mole percent based on the total number of moles ofcarboxylate and/or acid thereof present in the feedstock.

The dissolved salts in the feedstock are zirconium carboxylate saltsrather than zirconium halide salts, zirconium oxyhalide salts, zirconiumnitrate salts, or zirconium oxynitrate salts. Halide and nitrate anionsin the feedstock tend to result in the formation of zirconia-containingnanoparticles that are predominately of a monoclinic phase rather thanthe more desirable tetragonal or cubic phases. Further, carboxylatesand/or acids thereof tend to be more compatible with an organic matrixmaterial compared to halides and nitrates. Many feedstocks are free orsubstantially free of halides and nitrates. As used herein withreference to halides and nitrates, the term “substantially free” meansthat halides and nitrates are not intentionally added to the feedstockbut may be present as impurities in other components such as in thezirconium carboxylate salt. The feedstock contains no greater than 30millimolar, no greater than 20 millimolar, no greater than 10millimolar, no greater than 5 millimolar, no greater than 1 millimolar,or no greater than 0.5 millimolar halide or nitrate. That is, theconcentration of halide or nitrate in the feedstock is in the range of 0to 30 millimolar, in the range of 0 to 20 millimolar, in the range of 0to 10 millimolar, in the range of 0 to 5 millimolar, in the range of 0to 1 millimolar, or in the range of 0 to 0.05 millimolar.

The zirconium carboxylate salt is often zirconium acetate salt.Zirconium acetate can be represented by a formula such as ZrO_(((4-n)/2)^(n+)(CH₃COO⁻)_(n) where n is in the range of 1 to 2. The zirconium ionmay be present in a variety of structures depending, for example, on thepH of the feedstock. Methods of making zirconium acetate are described,for example, in W. B. Blumenthal, “The Chemical Behavior of Zirconium,”pp. 311-338, D. Van Nostrand Company, Princeton, N.J. (1958). Suitableaqueous solutions of zirconium acetate are commercially available, forexample, from Magnesium Elektron, Inc. (Flemington, N.J.) that containup to 17 weight percent zirconium, up to 18 weight percent zirconium, upto 20 weight percent zirconium, up to 22 weight percent, up to 24 weightpercent, up to 26 weight percent, or up to 28 weight percent zirconiumbased on the total weight of the solution.

As prepared from various commercially available sources, the zirconiumcarboxylate salt solutions often contain some alkali metal ions (e.g.,sodium ions, potassium ions, or lithium ions), some alkaline earth ions(e.g., calcium ions, magnesium ions, barium ions, or strontium ions), ora mixture thereof. In many applications, the removal of at least aportion of the alkali metal ions, alkaline earth ions, or both can bedesirable. For example, if these ions are present in the feedstock abovea certain low amount, the resulting zirconia-containing nanoparticlestend to be associated rather than non-associated. Surprisingly, theseions can be removed from a zirconium carboxylate salt solution bycontacting the solution with a cation exchange resin in a hydrogen form.That is, a solution containing a dissolved zirconium carboxylate saltcan be contacted with a cation exchange resin in a hydrogen form toremove alkali metal ions, alkaline earth ions, or a mixture thereof. Thealkali metal ions and alkaline earth ions can exchange with the hydrogenions sorbed on the cation exchange resin. These ions (i.e., cations) candisplace the hydrogen ions from the cation exchange resin.

It is unexpected that alkali metal ions, alkaline earth ions, or amixture thereof can be selectively removed from a zirconium carboxylatesalt solution. Normally, cation exchange resins are used to removehighly charged cations from a solution by sorbing them onto the cationexchange resin while cations with a lower charge are released back intothe solution. It is unexpected that a cation exchange resin wouldpreferentially sorb alkali metal ions or alkaline earth ions that arepresent in a relatively low concentration compared to zirconium ions,which may be a multivalent ion in solution, that are present in arelatively high concentration. That is, it is surprising that thecapacity of the cation exchange resin is not exhausted by the zirconiumions and it is surprising that there is capacity available to sorbalkali metal ions and alkaline earth ions from solution.

In some methods of contacting the zirconium carboxylate salt solutionwith a cation exchange resin in a hydrogen form, the cation exchangeresin can be added directly into the solution. This solution can be thefeedstock for the hydrothermal treatment. After removal of at least aportion of the alkali metal ions, alkaline earth ions, or both, thecation exchange resin is separated from the solution. For example, thecation exchange resin can be separated by filtration or centrifugation.

In other methods of contacting the zirconium carboxylate salt solutionwith a cation exchange resin in a hydrogen form, the cation exchangeresin can be placed in a chromatographic column or disposed on thesurface of a filtration medium. The filtration medium can be positioned,for example, within a housing to provide a filter cartridge. Suitablefiltration medium and systems that include a filter cartridge arefurther described, for example, in U.S. Pat. No. 5,468,847 (Heilmann etal.). The filtration medium can be in the form of vertical pleatedfilters such as those described in U.S. Pat. No. 3,058,594 (Hultgren).In other embodiments, the filtration medium is in the form ofhorizontal, compound radially pleated filters such as those described inU.S. Pat. No. 4,842,739 (Tang et al.).

Enough cation exchange resin in the hydrogen form is typically added toremove at least 50 mole percent of the alkali metal ions, alkalineearths, or both in the zirconium carboxylate solution. In someembodiments, at least 60 mole percent, at least 70 mole percent, atleast 80 mole percent, at least 90 mole percent, or at least 95 molepercent of the alkali metal ions, alkaline earth ions, or both areremoved by contacting the solution with the cation exchange resin. Theamount of cation exchange resin that is contacted with the solution canbe calculated from the ionic capacity of the cation exchange resin andthe total amount of alkali metal ions and alkaline earth ions dissolvedin the solution.

In practice, an excess of the cation exchange resin is contacted withthe zirconium carboxylate salt solution. For example, the concentrationof the alkali ions and alkaline earth ions in the solution can bedetermined using a technique such as Inductively Coupled Plasma AtomicEmission Spectroscopy. The capacity of the cation exchange resin isoften supplied by the manufacturer but can also be calculated bydisplacing all of the hydrogen ions in a sample of the cation exchangeresin with another ion such as sodium or calcium and then titrating thetotal amount of hydrogen ions displaced. Based on the capacity of thecation exchange resin and the concentration of alkali metal ions andalkaline earth ions in the zirconium carboxylate salt solution, at leasta 10 mole percent excess of the cation exchange resin is contacted withthe solution. In many embodiments, the cation exchange resin is presentin an excess of at least 20 mole percent, at least 50 mole percent, atleast 75 mole percent, at least 100 mole percent, or at least 200 molepercent based on the total amount of alkali metal ions and alkalineearth ions in the zirconium carboxylate salt solution.

Any known cation exchange resin in the hydrogen form can be used. Forexample, some suitable cation exchange resins are commercially availablefrom Rohm and Haas Company (Philadelphia, Pa.) under the tradedesignation AMBERLITE such as AMBERLITE IR-120. Other suitable cationexchange resins are commercially available from Dow Chemical (Midland,Mich.) under the trade designation DOWEX such as DOWEX G-26. Still othersuitable cation exchange resins are commercially available from PuroliteCompany (Bala Cynwyd, Pa.) under the trade designation PUROLITE such asPUROLITE C160H. Alternatively, cation exchange resins in a sodium formcan be converted to a hydrogen form by contacting them with diluteacids. Any suitable mesh size of the cation exchange resin can be used.In some embodiments, the mesh size is in the range of 16 to 200 mesh, inthe range of 16 to 100 mesh, or in the range of 16 to 50 mesh.

In many feedstocks, either with or without treatment of the zirconiumcarboxylate salt solution with a cation exchange resin in a hydrogenform, the total concentration of alkali metal ions is no greater than 3milligrams per gram of zirconium. For example, the total concentrationof alkali metal ions is often no greater than 2.5 milligrams per gram ofzirconium, no greater than 2.0 milligrams per gram of zirconium, nogreater than 1.5 milligrams per gram of zirconium, no greater than 1.0milligram per gram of zirconium, no greater than 0.5 milligrams per gramof zirconium, no greater than 0.3 milligrams per gram of zirconium, nogreater than 0.2 milligrams per gram of zirconium, or no greater than0.1 milligrams per gram of zirconium.

Often, after treatment of the zirconium carboxylate salt solution with acation exchange resin in a hydrogen form, the total concentration ofalkali metal ions in the feedstock is not greater than 1 milligram pergram of zirconium, no greater than 0.6 milligram per gram of zirconium,no greater than 0.5 milligrams per gram of zirconium, no greater than0.3 milligrams per gram of zirconium, no greater than 0.2 milligrams pergram of zirconium, or no greater than 0.1 milligrams per gram ofzirconium.

Similarly, in many feedstocks, either with or without treatment of thezirconium carboxylate salt solution with a cation exchange resin in ahydrogen form, the total concentration of alkaline earth ions is nogreater than 3 milligrams per gram of zirconium. For example, thealkaline earth ion content is often no greater than 2.5 milligrams pergram of zirconium, no greater than 2.0 milligrams per gram of zirconium,no greater than 1.5 milligrams per gram of zirconium, no greater than1.0 milligram per gram of zirconium, no greater than 0.5 milligrams pergram of zirconium, no greater than 0.3 milligrams per gram of zirconium,no greater than 0.2 milligrams per gram of zirconium, or no greater than0.1 milligrams per gram of zirconium.

Often, after treatment of the zirconium carboxylate salt solution with acation exchange resin in a hydrogen form, the concentration of alkalineearth ions in the feedstock is not greater than 1 milligram per gram ofzirconium, no greater than 0.6 milligrams per gram of zirconium, nogreater than 0.5 milligrams per gram of zirconium, no greater than 0.3milligrams per gram of zirconium, no greater than 0.2 milligrams pergram of zirconium, or no greater than 0.1 milligrams per gram ofzirconium.

In applications where the zirconia-containing nanoparticles aresuspended or dispersed in an organic matrix, it can be desirable thatthe total amount of alkali metal ions in the feedstock is less than 0.6milligrams per gram of zirconium and that the total amount of alkalineearth ions in the feedstock is less than 0.6 milligrams per gram ofzirconium. If the total amount of alkali metal ions, alkaline earthions, or both exceed this amount, there is an increased tendency for theresulting zirconia-containing nanoparticles to be aggregated oragglomerated. In many examples, the total amount of alkali metal ions inthe feedstock is less than 0.5 milligrams per gram of zirconium, lessthan 0.3 milligrams per gram of zirconium, or less than 0.1 milligramper gram of zirconium and the total amount of alkaline earth ions in thefeedstock is less than 0.5 milligrams per gram of zirconium, less than0.3 milligrams per gram of zirconium, or less than 0.1 milligram pergram of zirconium.

Some feedstocks contain a dissolved yttrium salt in addition to thedissolved zirconium carboxylate salt. As with the zirconium carboxylatesalt, the anion of the yttrium salt is typically chosen to be removableduring subsequent processing steps, to be non-corrosive, and to becompatible with an organic matrix. The yttrium salt is often yttriumcarboxylate with the carboxylate having no more than four carbon atoms.In many embodiments, the carboxylate is acetate. The yttrium is oftenpresent in an amount up to 20 weight percent based on a total weight ofyttrium and zirconium in the feedstock. For example, the yttrium isoften present in an amount up to 18 weight percent, up to 15 weightpercent, up to 12 weight percent, up to 10 weight percent, up to 8weight percent, up to 6 weight percent, or up to 4 weight percent basedon the total weight of yttrium and zirconium in the feedstock. That is,the amount of yttrium in the feedstock is often in the range of 0 to 20weight percent, 1 to 20 weight percent, 1 to 18 weight percent, 1 to 10weight percent, or 1 to 6 weight percent based on the total weight ofyttrium and zirconium.

Expressed differently, the weight ratio of yttrium to zirconium (i.e.,grams yttrium÷grams zirconium) in the feedstock is often up to 0.25, upto 0.22, up to 0.20, up to 0.16, up to 0.12, up to 0.08. For example,the weight ratio of yttrium to zirconium can be in the range of 0 to0.25, 0 to 0.22, 0.01 to 0.22, 0.02 to 0.22, 0.04 to 0.22, 0.04 to 0.20,0.04 to 0.16, or 0.04 to 0.12.

The pH of the feedstock is typically acidic. For example, the pH isusually less than 6, less than 5, or less than 4. The pH often is in therange of 3 to 4.

The liquid phase of the feedstock is typically predominantly water(i.e., the liquid phase is an aqueous based medium). This water ispreferably deionized to minimize the introduction of alkali metal ions,alkaline earth ions, or both into the feedstock. Water-miscible organicco-solvents can be included in the liquid phase in amounts up 20 weightpercent based on the weight of the liquid phase. Suitable co-solventsinclude, but are not limited to, 1-methoxy-2-propanol, ethanol,isopropanol, ethylene glycol, N,N-dimethylacetamide, andN-methylpyrrolidone.

Typically, the feedstock is a solution and does not contain dispersed orsuspended solids. For example, seed particles usually are not present inthe feedstock. The feedstock typically contains greater than 5 weightpercent solids and these solids are typically dissolved. As used herein,the “weight percent solids” is calculated by drying a sample at 120° C.and refers the portion of the feedstock that is not water, awater-miscible co-solvent, or another compound that can be vaporized attemperatures up to 120° C. The weight percent solids is equal to100(wet weight−dry weight)÷(wet weight).In this equation, the term “wet weight” refers to the weight of afeedstock sample before drying and the term “dry weight” refers to theweight of the sample after drying, for example, at 120° C. for at least30 minutes. If the feedstock has percent solids greater than 5 weightpercent, the resulting zirconia-containing nanoparticles are typicallynon-associated. Surprisingly, however, if the feedstock has percentsolids equal to or less than 5 weight percent, the resultingzirconia-containing nanoparticles are typically associated. This isunexpected because the conventional approach to forming non-associatedparticles is to lower the concentration of the reactants introduced intoa hydrothermal reactor.

In many embodiments, the feedstock contains greater than 6 weightpercent, greater than 7 weight percent, greater than 8 weight percent,greater than 10 weight percent, greater than 12 weight percent, greaterthan 14 weight percent, greater than 16 weight percent, greater than 18weight percent, greater than 20 weight percent solids, or greater than24 weight percent solids. Some feedstocks contain up to 47 weightpercent, which corresponds to concentrations of zirconium acetate thatis commercially available. For example, the feedstock can contain up to45 weight percent, up to 40 weight percent, up to 36 weight percent, upto 32 weight percent, up to 30 weight percent, up to 28 weight percent,up to 25 weight percent, up to 24 weight percent, up to 23 weightpercent, or up to 20 weight percent solids. Some exemplary feedstockshave solids in the range of 6 to 47 weight percent, 6 to 40 weightpercent, 6 to 37 weight percent, 8 to 36 weight percent, 8 to 30 weightpercent, 8 to 24 weight percent, or 8 to 20 weight percent.

Expressed differently, the feedstock typically contains greater than 2.2weight percent zirconium based on the weight of the feedstock. Someexemplary feedstocks have a zirconium content in the range of 2.6 to20.7 weight percent, in the range of 2.6 to 16.3 weight percent, in therange of 3.5 to 15.8 weight percent, in the range of 3.5 to 11.0 weightpercent, in the range of 3.5 to 10.6 weight percent, or in the range of3.5 to 8.8 weight percent zirconium based on the weight of thefeedstock.

A feedstock containing 5 weight percent solids often contains about 0.36to 0.42 mmoles carboxylic acid and/or anion thereof per gram ofsolution, which can correspond to about 2.2 to 2.6 weight percent aceticacid and/or acetate. Similarly a feedstock containing 8 weight percentsolids often contains about 0.58 to 0.68 mmoles of carboxylic acidand/or anion thereof per gram of solution, which can correspond to about3.5 to 4.1 weight percent acetic acid and/or acetate. A feedstockcontaining 10 weight percent solids often contains about 0.72 to 0.84mmoles of carboxylic acid and/or anion thereof per gram of solution,which can correspond to about 4.3 to 5.0 weight percent acetic acidand/or acetate. A feedstock containing 17 weight percent solids oftencontains about 1.22 to 1.42 mmoles of carboxylic acid and/or anionthereof per gram of solution, which can correspond to about 7.3 to 8.5weight percent acetic acid and/or acetate. A feedstock containing 19weight percent solids often contains about 1.37 to 1.6 mmoles ofcarboxylic acid and/or anion thereof per gram of solution, which cancorrespond to about 8.2 to 9.5 weight percent acetic acid and/oracetate. A feedstock containing 25 weight percent solids often containsabout 1.8 to 2.1 mmoles of carboxylic acid and/or anion thereof per gramof solution, which can correspond to about 10.8 to 12.6 weight percentacetic acid and/or acetate. Further, a feedstock containing 45 weightpercent solids often contains about 3.2 to 3.7 mmoles of carboxylic acidand/or anion thereof per gram of solution, which can correspond to about19.4 to 22.4 weight percent acetic acid and/or acetate.

The feedstock is subjected to a single hydrothermal treatment. Thedissolved zirconium species in the feedstock undergoes hydrolysis andcondensation to form a zirconia-containing nanoparticle. At least 90weight percent of the dissolved zirconium in the feedstock undergoeshydrolysis and condensation with the single hydrothermal treatment. Asused herein, the phrase “single hydrothermal treatment” means that anintermediate is not isolated from the hydrolysis and condensationreaction that is less than 90 weight percent converted tozirconia-containing nanoparticles. The byproducts of the hydrolysis andcondensation reactions are usually not removed until at least 90 weightpercent of the zirconium in the feedstock has been converted tozirconia-containing nanoparticles. In some embodiments, at least 92weight percent, at least 95 weight percent, at least 97 weight percent,at least 98 weight percent, at least 99 weight percent, or 100 weightpercent of the dissolved zirconium in the feedstock undergoes hydrolysisand condensation during the single hydrothermal treatment.

The percent conversion (i.e., the extent of hydrolysis and condensation)can be calculated, for example, using Thermal Gravimetric Analysis(TGA). The percent conversion of the zirconia-containing sample underanalysis can be given by the following equation% Conversion=100(A−B)/(A−C)where A is a percent weight loss of the feedstock, B is a percent weightloss of the zirconia-containing sample under analysis, and C is apercent weight loss of a zirconia-containing standard known or believedto be completely converted. The percent weight loss for the feedstock,the zirconia-containing sample under analysis, and thezirconia-containing standard are determined by drying each sample (e.g.,3 to 6 grams) at 120° C. for 30 minutes before analysis. Afterequilibration at 85° C. in the thermal gravimetric analyzer, each sampleis heated at a rate of 20° C./minute to 200° C. The temperature is heldat 200° C. for 20 minutes, increased at a rate of 20° C./minute to 900°C., and held at 900° C. for 20 minutes. The percent weight loss can becalculated from the following equation% weight loss=100(weight_(200C)−weight_(900C))/weight_(900C)for the first feedstock, the zirconia-containing sample under analysis,and the zirconia-containing standard. The percent weight losscorresponds to what is not an inorganic oxide in each of the samplesused in the analysis.

Any optional yttrium species in the feedstock can also undergohydrolysis and condensation along with the zirconium species. Thehydrolysis and condensation reactions of the zirconium and the optionalyttrium are often accompanied with the release of an acidic byproduct.That is, the byproduct is often a carboxylic acid corresponding to thecarboxylate of the zirconium carboxylate salt and any optional yttriumcarboxylate salt. For example, if the carboxylate in the salt isformate, acetate, propionate, or butyrate, then the byproduct typicallycontains formic acid, acetic acid, propionic acid, or butyric acid,respectively.

The single hydrothermal treatment can be in a batch reactor or in acontinuous reactor. The heating times are typically shorter and thetemperatures are typically higher in a continuous hydrothermal reactorcompared to a batch hydrothermal reactor. The time of the hydrothermaltreatments can be varied depending on the type of reactor, thetemperature of the reactor, and the concentration of the feedstock. Thepressure in the reactor can be autogeneous (i.e., the vapor pressure ofwater at the temperature of the reactor), can be hydraulic (i.e., thepressure caused by the pumping of a fluid against a restriction), or canresult from the addition of an inert gas such as nitrogen or argon.Suitable batch hydrothermal reactors are available, for example, fromParr Instruments Co. (Moline, Ill.). Some suitable continuoushydrothermal reactors are described, for example, in U.S. Pat. Nos.5,453,262 (Dawson et al.) and 5,652,192 (Matson et al.); Adschiri etal., J. Am. Ceram. Soc., 75, 1019-1022 (1992); and Dawson, CeramicBulletin, 67 (10), 1673-1678 (1988).

If a batch reactor is used for the single hydrothermal treatment to formzirconia-containing nanoparticles, the temperature is often in the rangeof 160° C. to 275° C., in the range of 160° C. to 250° C., in the rangeof 170° C. to 250° C., in the range of 175° C. to 250° C., in the rangeof 175° C. to 225° C., in the range of 180° C. to 220° C., in the rangeof 180° C. to 215° C., or in the range of 190° C. to 210° C. Thefeedstock is typically placed in the batch reactor at room temperature.The feedstock within the batch reactor is heated to the designatedtemperature and held at that temperature for at least 30 minutes, atleast 1 hour, at least 2 hours, or at least 4 hours. The temperature canbe held up to 24 hours, up to 20 hours, up to 16 hours, or up to 8hours. For example, the temperature can be held in the range of 0.5 to24 hours, in the range of 1 to 18 hours, in the range of 1 to 12 hours,or in the range of 1 to 8 hours. Any size batch reactor can be used. Forexample, the volume of the batch reactor can be in a range of severalmilliliters to several liters or more.

In many embodiments, the feedstock is passed through a continuoushydrothermal reactor for the single hydrothermal treatment. As usedherein, the term “continuous” with reference to the hydrothermal reactorsystem 100 means that the feedstock 110 is continuously introduced andan effluent is continuously removed from the heated zone. Theintroduction of feedstock and the removal of the effluent typicallyoccur at different locations of the reactor. The continuous introductionand removal can be constant or pulsed. It is surprising that thefeedstock can be passed through a continuous hydrothermal reactorbecause the feedstock often thickens and forms a gel when heated.Conventional wisdom would suggest that such a feedstock should not bepassed through a continuous hydrothermal reactor because of a concernthat the material could become too thick to pump or could result in theformation of a plug within the reactor. Conventional wisdom also wouldsuggest that a feedstock with high weight percent solids such as 10weight percent or 12 weight percent or higher could not be pumpedwithout plugging the reactor system.

One exemplary continuous hydrothermal reactor system 100 is shownschematically in FIG. 1. The feedstock 110 is contained within afeedstock tank 115. The feedstock tank is connected with tubing orpiping 117 to a pump 120. Similar tubing or piping can be used toconnect other components of the tubular reactor system. The tubing orpiping 117 can be constructed of any suitable material such as metal,glass, ceramic, or polymer. The tubing or piping 117 can be, forexample, polyethylene tubing or polypropylene tubing in the portions ofthe continuous hydrothermal reactor system 100 that are not heated andthat are not under high pressure. Any tubing that is heated or underpressure is often made of metal (e.g., stainless steel, carbon steel,titanium, nickel, or the like) or has a metal outer housing. The pump120 is used to introduce the feedstock 110 into the tubular reactor 130.That is, the pump 120 is connected to the inlet of the tubular reactor130. Any type of pump 120 can be used that is capable of pumping againstthe pressure within the tubular reactor 130. The pump can provide aconstant or pulsed flow of the feedstock solution into the tubularreactor 130.

As used herein, the term “tubular reactor” refers to the portion of thecontinuous hydrothermal reactor system that is heated (i.e., the heatedzone). Although the tubular reactor 130 is shown in FIG. 1 as a coil oftubing, the tubular reactor can be in any suitable shape. The shape ofthe tubular reactor is often selected based on the desired length of thetubular reactor and the method used to heat the tubular reactor. Forexample, the tubular reactor can be straight, U-shaped, or coiled. Theinterior portion of the tubular reactor can be empty or can containbaffles, balls, or other known mixing means.

As shown in FIG. 1, the tubular reactor 130 is placed in a heatingmedium 140 within a heating medium vessel 150. The heating medium 140can be, for example, an oil, sand, salt, or the like that can be heatedto a temperature above the hydrolysis and condensation temperatures ofthe zirconium. Suitable oils include, for example, plant oils such aspeanut oil and canola oil. Some plant oils are preferably kept undernitrogen when heated to prevent or minimize oxidation of the oils. Othersuitable oils include polydimethylsiloxanes such as those commerciallyavailable from Duratherm Extended Fluids (Lewiston, N.Y.) under thetrade designation “DURATHERM S”. Suitable salts include, for example,sodium nitrate, sodium nitrite, potassium nitrate, or mixtures thereof.The heating medium vessel 150 can be any suitable container that canhold the heating medium and that can withstand the heating temperaturesused for the tubular reactor 130. The heating medium vessel 150 can beheated using any suitable means. In many embodiments, the heating mediumvessel 150 is positioned inside an electrically heated coil.Alternatively, other types of heaters such as, for example, inductionheaters, microwave heaters, fuel-fired heaters, heating tape, and steamcoils can be used in place of the heating vessel 150, the heating medium140, or both.

The tubular reactor 130 can be made of any material capable ofwithstanding the temperatures and pressures used to prepare zirconiananoparticles. The tubular reactor 130 preferably is constructed of amaterial that can resist dissolution in an acidic environment. Forexample, carboxylic acids can be present in the feedstock or can beproduced as a reaction byproduct within the continuous hydrothermalreactor system. In some exemplary embodiments, the tubular reactor ismade of stainless steel, nickel, titanium, carbon-based steel, or thelike.

In other exemplary embodiments, an interior surface of the tubularreactor contains a fluorinated polymeric material. This fluorinatedpolymeric material can include, for example, a fluorinated polyolefin.In some embodiments, the polymeric material is polytetrafluoroethylene(PTFE) such as TEFLON, which is a trade designation of DuPont(Wilmington, Del.). Some tubular reactors have a fluorinated polymerichose such as a TEFLON hose within a metal housing such as a braidedstainless steel housing. The fluorinated polymeric surface isparticularly advantageous for use with feedstocks and/or reactionproducts that contain carboxylic acids. These carboxylic acids can leachmetals from some known hydrothermal reactors such as those constructedof stainless steel.

Surprisingly, the heat transfer is typically sufficient through thefluorinated polymeric material to convert the zirconium in the feedstockto zirconia-containing nanoparticle under continuous hydrothermalconditions. Typical values of the thermal conductivities of 316Lstainless steel and PTFE are 18 W/(m K) and 0.25 W/(m K), respectively.When made into tubing, a typical wall thickness of 0.00089 meters for0.25 inch stainless steel tubing will withstand hydrothermal conditions.This results in a heat-transfer coefficient of 20,000 W/(m²K) for such astainless steel tube. A typical wall thickness of a 0.25 inch PTFE tubeis 0.0010 meters. This results in a heat-transfer coefficient of 250W/(m²K) for such a tube. The value for the stainless steel tube is 80times that of the PTFE tube. Since a PTFE tube cannot withstand thepressures of a hydrothermal reaction, it needs to be encased in asheath, for example stainless steel braiding, designed to contain thepressure. Such a sheath has a typical thickness of 0.0023 meters.Although the resistance to heat transfer of such a braided sheath isdifficult to estimate, it will add an amount to the resistance of thePTFE tube itself, making the advantage of the stainless steel tube evengreater. Consequently, it is surprising that this type of compositetubing will work in a hydrothermal reactor that requires a considerableability to exchange energy between the contents of the tube and theexternal environment.

The second end of the tubular reactor 130 is usually connected to acooling device 160. Any suitable cooling device 160 can be used. In someembodiments, the cooling device 160 is a heat exchanger that includes asection of tubing or piping that has an outer jacket filled with acooling medium such as cool water. In other embodiments, the coolingdevice 160 includes a coiled section of tubing or piping that is placedin a vessel that contains cooling water. In either of these embodiments,the tubular reactor effluent is passed through the section of tubing andis cooled from the tubular reactor temperature to a temperature nogreater than 100° C., no greater than 80° C., no greater than 60° C., orno greater than 40° C. Other cooling devices that contain dry ice orrefrigeration coils can also be used. After cooling, the reactoreffluent can be discharged into a product collection vessel 180. Thereactor effluent is preferably not cooled below the freezing point priorto being discharged into the product collection vessel 180.

The pressure inside the tubular reactor can be at least partiallycontrolled with a backpressure valve 170, which is generally positionedbetween the cooling device 160 and the sample collection vessel 180. Thebackpressure valve 170 controls the pressure at the exit of thecontinuous hydrothermal reactor system 100 and helps to control thepressure within the tubular reactor 130. The backpressure is often atleast 100 pounds per square inch (0.7 MPa), at least 200 pounds persquare inch (1.4 MPa), at least 300 pounds per square inch (2.1 MPa), atleast 400 pounds per square inch (2.8 MPa), at least 500 pounds persquare inch (3.5 MPa), at least 600 pounds per square inch (4.2 MPa), orat least 700 pounds per square inch (4.9 MPa). The backpressure shouldbe high enough to prevent boiling within the tubular reactor.

The dimensions of the tubular reactor 130 can be varied and, inconjunction with the flow rate of the feedstock, can be selected toprovide suitable residence times for the reactants within the tubularreactor. Any suitable length tubular reactor can be used provided thatthe residence time and temperature are sufficient to convert thezirconium in the feedstock to zirconia-containing nanoparticles. Thetubular reactor often has a length of at least 0.5 meter, at least 1meter, at least 2 meters, at least 5 meters, at least 10 meters, atleast 15 meters, at least 20 meters, at least 30 meters, at least 40meters, or at least 50 meters. The length of the tubular reactor in someembodiments is less than 500 meters, less than 400 meters, less than 300meters, less than 200 meters, less than 100 meters, less than 80 meters,less than 60 meters, less than 40 meters, or less than 20 meters.

Tubular reactors with a relatively small inner diameter are typicallypreferred. For example, tubular reactors having an inner diameter nogreater than about 3 centimeters are often used because of the fast rateof heating of the feedstock that can be achieved with these reactors.Also, the temperature gradient across the tubular reactor is less forreactors with a smaller inner diameter compared to those with a largerinner diameter. The larger the inner diameter of the tubular reactor,the more this reactor resembles a batch reactor. However, if the innerdiameter of the tubular reactor is too small, there is an increasedlikelihood of the reactor becoming plugged or partially plugged duringoperation resulting from deposition of material on the walls of thereactor. The inner diameter of the tubular reactor is often at least 0.1centimeters, at least 0.15 centimeters, at least 0.2 centimeters, atleast 0.3 centimeters, at least 0.4 centimeters, at least 0.5centimeters, or at least 0.6 centimeters. In some embodiments, thediameter of the tubular reactor is no greater than 3 centimeters, nogreater than 2.5 centimeters, no greater than 2 centimeters, no greaterthan 1.5 centimeters, or no greater than 1.0 centimeters. Some tubularreactors have an inner diameter in the range of 0.1 to 3.0 centimeters,in the range of 0.2 to 2.5 centimeters, in the range of 0.3 to 2centimeters, in the range of 0.3 to 1.5 centimeters or in the range of0.3 to 1 centimeters.

Rather than increasing the inner diameter of the tubular reactor, it maybe preferable to use multiple tubular reactors having a smaller innerdiameter arranged in a parallel manner. For example, rather thanincreasing the inner diameter of the tubular reactor to produce a largeramount of zirconia-containing nanoparticles, multiple tubular reactorshaving an inner diameter no greater than about 3 centimeters can beoperated in a parallel manner.

In a continuous hydrothermal reactor, the temperature and the residencetime are selected in conjunction with the tubular reactor dimensions toconvert at least 90 weight percent of the zirconium in the feedstock tozirconia-containing nanoparticles using a single hydrothermal treatment.That is, at least 90 weight percent of the dissolved zirconium in thefeedstock is converted to zirconia-containing nanoparticles within asingle pass through the continuous hydrothermal reactor system.

When referring to a continuous hydrothermal reactor, the term “residencetime” means the average length of time that the feedstock is within theheated portion of the continuous hydrothermal reactor system. For thereactor depicted in FIG. 1, the residence time is the average time thefeedstock is within the tubular reactor 130 and is equal to the volumeof the tubular reactor divided by the flow rate of the feedstock throughthe tubular reactor. The residence time in the tubular reactor can bevaried by altering the length or diameter of the tubular reactor as wellas by altering the flow rate of the feedstock. In many embodiments, theresidence time is at least 1 minute, at least 2 minutes, at least 4minutes, at least 6 minutes, at least 8 minutes, or at least 10 minutes.The residence time is typically no greater than 240 minutes, no greaterthan 180 minutes, no greater than 120 minutes, no greater than 90minutes, no greater than 60 minutes, no greater than 45 minutes, or nogreater than 30 minutes. In many examples, the residence time is in therange of 1 to 240 minutes, in the range of 1 to 180 minutes, in therange of 1 to 120 minutes, in the range of 1 to 90 minutes, in the rangeof 1 to 60 minutes, in the range of 10 to 90 minutes, in the range of 10to 60 minutes, in the range of 20 to 60 minutes, or in the range of 30to 60 minutes.

Any suitable flow rate of the feedstock through the tubular reactor canbe used as long as the residence time is sufficiently long to convertthe dissolved zirconium to zirconia-containing nanoparticles. That is,the flow rate is often selected based on the residence time needed toconvert the zirconium in the feedstock to zirconia-containingnanoparticles. Higher flow rates are desirable for increasing throughputand for minimizing the deposition of materials on the walls of thetubular reactor. A higher flow rate can often be used when the length ofthe reactor is increased or when both the length and diameter of thereactor are increased. The flow through the tubular reactor can beeither laminar or turbulent.

In some exemplary continuous hydrothermal reactors, the reactortemperature is in the range of 170° C. to 275° C., in the range of 170°C. to 250° C., in the range of 170° C. to 225° C., in the range of 180°C. to 225° C., in the range of 190° C. to 225° C., in the range of 200°C. to 225° C., or in the range of 200° C. to 220° C. If the temperatureis greater than about 275° C., the pressure may be unacceptably high forsome hydrothermal reactors systems. However, if the temperature is lessthan about 170° C., the conversion of the zirconium in the feedstock tozirconia-containing nanoparticles may be less than 90 weight percentusing typical residence times.

The effluent of the hydrothermal treatment (i.e., the product of thehydrothermal treatment) contains zirconia-containing nanoparticles. Moreparticularly, the effluent of the hydrothermal treatment is azirconia-containing sol. As used herein, the term “sol” refers to adispersion or suspension of the zirconia-containing nanoparticles in anaqueous-based medium.

In many applications, at least a portion of the aqueous-based medium isremoved from the zirconia-containing sol. Any known means for removingthe aqueous-based medium can be used. This aqueous-based medium containswater and often contains dissolved carboxylic acids and/or anionsthereof that are present in the feedstock or that are byproducts of thereactions that occur within the hydrothermal reactor. As used herein,the term “carboxylic acids and/or anions thereof” refers to carboxylicacids, carboxylate anions of these carboxylic acids, or mixturesthereof. The removal of at least a portion of these dissolved carboxylicacids and/or anions thereof from the zirconia-containing sol may bedesirable in some applications. The zirconia-containing sol can besubjected to methods such as vaporization, drying, ion exchange, solventexchange, diafiltration, or dialysis.

In some embodiments, the effluent of the hydrothermal reactor isconcentrated or dried with a drying process. Along with removing atleast a portion of the water present in the effluent, the concentrationprocess often results in the vaporization of at least a portion of thedissolved carboxylic acids. Any suitable drying method can be used suchas spray drying, gap drying, or oven drying. For example, the effluentcan be dried in a conventional oven at a temperature of at least 80° C.,at least 90° C., at least 100° C., at least 110° C., or at least 120° C.The drying time is often greater than 1 hour, greater than 2 hours, orgreater than 3 hours.

In other embodiments, the effluent of the hydrothermal treatment issubjected to a solvent exchange process. An organic solvent with ahigher boiling point than water can be added to the effluent. Examplesof organic solvents that are suitable for use in a solvent exchangemethod include, but are not limited to, 1-methoxy-2-propanol andN-methylpyrrolidone. The mixture containing the effluent plus theorganic solvent can be treated to remove the water using methods suchas, for example, distillation, rotary evaporation, or oven drying.Often, at least a portion of the dissolved carboxylic acids can beremoved along with the water.

In other embodiments, the effluent of the hydrothermal treatment can besubjected to dialysis or diafiltration. Dialysis and diafiltration bothtend to remove at least a portion of the dissolved carboxylic acidsand/or anions thereof. For dialysis, a sample of the effluent can bepositioned within a membrane bag that is closed and then placed within awater bath. The carboxylic acid and/or carboxylate anions diffuse out ofthe sample within the membrane bag. That is, these species will diffuseout of the effluent through the membrane bag into the water bath toequalize the concentration within the membrane bag to the concentrationin the water bath. The water in the bath is typically replaced severaltimes to lower the concentration of species within the bag. A membranebag is typically selected that allows diffusion of the carboxylic acidsand/or anions thereof but does not allow diffusion of thezirconia-containing nanoparticles out of the membrane bag.

For diafiltration, a permeable membrane is used to filter the sample.The zirconia particles can be retained on the filter if the pore size ofthe filter is appropriately chosen. The dissolved carboxylic acidsand/or anions thereof pass through the filter. Any liquid that passesthrough the filter is replaced with fresh water. In a discontinuousdiafiltration process, the sample is often diluted to a pre-determinedvolume and then concentrated back to the original volume byultrafiltration. The dilution and concentration steps are repeated oneor more times until the carboxylic acid and/or anions thereof areremoved or lowered to an acceptable concentration level. In a continuousdiafiltration process, which is often referred to as a constant volumediafiltration process, fresh water is added at the same rate that liquidis removed through filtration. The dissolved carboxylic acid and/oranions thereof are in the liquid that is removed.

In yet another embodiment, the effluent of the hydrothermal treatmentcan be contacted with an anion exchange resin in a hydroxyl form. Byadjusting the pH of the effluent, the carboxylic acids can be convertedto the basic form (i.e., carboxylate anion). At least some of thecarboxylate anions can replace some of the hydroxyl ions on the anionexchange resin. The pH adjusted effluent can be passed through a columncontaining the anion exchange resin or through a filtration medium thatincludes the anion exchange resin. Alternatively, the anion exchangeresin can be mixed with the effluent of the continuous hydrothermalreactor. After ion exchange, the anion exchange resin can be removed byfiltration. The size of the anion exchange resin is selected so that itcan be easily filtered from the treated effluent. For example, the sizeof the anion exchange resin is often no less than 200 mesh, no less than100 mesh, or no less than 50 mesh.

The zirconia-containing nanoparticles can optionally contain yttrium.Any yttrium that is present is typically in the form of yttrium oxide.The presence of yttrium in the zirconia-containing nanoparticle usuallyfacilitates the formation of the cubic/tetragonal phases rather than themonoclinic phase. The cubic and tetragonal phases are often preferredbecause they tend to have a higher refractive index and x-ray opacitycompared to the monoclinic phase. These phases also tend to be moresymmetrical, which can be an advantage in some applications when thezirconia-containing nanoparticles are suspended or dispersed in anorganic matrix because they have a minimal effect on the viscosity ofthe organic matrix. Additionally, the percent loading can be higher withthe cubic and tetragonal phases.

The mole ratio of yttrium to zirconium (i.e., moles yttrium÷moleszirconium) in the zirconia-containing nanoparticles is often up to 0.25,up to 0.22, up to 0.20, up to 0.16, up to 0.12, up to 0.08. For example,the mole ratio of yttrium to zirconium can be in the range of 0 to 0.25,0 to 0.22, 0.01 to 0.22, 0.02 to 0.22, 0.04 to 0.22, 0.04 to 0.20, 0.04to 0.16, or 0.04 to 0.12.

Expressed differently as oxides, the zirconia-containing nanoparticlesoften contain up to 11 mole percent Y₂O₃ based on the moles of theinorganic oxides (i.e., Y₂O₃ plus ZrO₂). For example, thezirconia-containing nanoparticles can contain up to 10 mole percent, upto 8 mole percent, up to 6 mole percent, or up to 4 mole percent Y₂O₃based on the moles of the inorganic oxides. Some zirconia-containingnanoparticles contain 0 to 11 mole percent, 0 to 10 mole percent, 1 to10 mole percent, 1 to 8 mole percent, or 2 to 8 mole percent Y₂O₃ basedon the moles of the inorganic oxides.

Expressed in yet another manner, the zirconia-containing nanoparticlesoften contain up to 20 weight percent Y₂O₃ based on the weight of theinorganic oxides (i.e., Y₂O₃ plus ZrO₂). For example, thezirconia-containing nanoparticles can contain up to 18 weight percent,up to 16 weight percent, up to 12 weight percent, up to 10 weightpercent, or up to 6 weight percent Y₂O₃ based on the weight of theinorganic oxides. Some zirconia-containing nanoparticles contain 0 to 20weight percent, 0 to 18 weight percent, 2 to 18 weight percent, 2 to 16weight percent, or 2 to 10 weight percent Y₂O₃ based on the weight ofthe inorganic oxides.

The zirconia-containing nanoparticles often contain at least someorganic material in addition to inorganic oxides. The organic materialcan be attached to the surface of the zirconia particles and oftenoriginates from the carboxylate species (anion, acid, or both) includedin the feedstock or formed as a byproduct of the hydrolysis andcondensation reactions. That is, the organic material is often sorbed tothe surface of the zirconia-containing nanoparticles. The zirconiaparticles often contain up to 15 weight percent, up to 12 weightpercent, up to 10 weight percent, up to 8 weight percent, or up to 6weight percent organic material based on the weight of the particles.

The zirconia-containing nanoparticles often contain less than 3milligrams of an alkali metal such as sodium, potassium, or lithium pergram of zirconium in the nanoparticles. For example, the amount ofalkali metal can be less than 2 milligrams per gram of zirconium, lessthan 1 milligram per gram of zirconium, less than 0.6 milligram per gramof zirconium, less than 0.5 milligram per gram of zirconium, less than0.3 milligram per gram of zirconium, less than 0.2 milligrams per gramof zirconium, or less than 0.1 milligram per gram of zirconium.

Likewise, the zirconia-containing nanoparticles often contain less than3 milligrams of an alkaline earth such as calcium, magnesium, barium, orstrontium per gram of zirconium in the nanoparticles. For example, theamount of alkaline earth can be less than 2 milligrams per gram ofzirconium, less than 1 milligram per gram of zirconium, less than 0.6milligram per gram of zirconium, less than 0.5 milligram per gram ofzirconium, less than 0.3 milligrams per gram of zirconium, less than 0.2milligrams per gram of zirconium, or less than 0.1 milligram per gram ofzirconium.

The effect of alkali metal ions and alkaline earth ions in the feedstockcan be seen in the appearance of the effluent from the hydrothermalreactor. If the amount of alkali metal ions or alkaline earth ions inthe feedstock is relatively high, the zirconia-containing sol tends tolook hazy rather than clear. Further, composite materials that areformed from effluents that appear hazy often have a higher relativeviscosity compared to composite materials that are prepared fromeffluents that appear clear. That is, the presence of alkaline metalions, alkaline earth ions, or both in the feedstock used to prepare thezirconia-containing nanoparticles can affect the viscosity of thecomposite materials.

The zirconia-containing nanoparticles are crystalline. Crystallinezirconia tends to have a higher refractive index and higher x-rayscattering capability than amorphous zirconia. Due to the difficulty inseparately quantifying cubic and tetragonal crystal structures for smallparticles using x-ray diffraction (i.e., the (1 1 1) peak for cubiczirconia often overlaps with the (1 0 1) peak for tetragonal zirconia),these two crystal structures are combined. For example, as shown in FIG.3 for exemplary zirconia-containing nanoparticles, the combination ofthese two peaks appears at about 30.5 degrees (2θ) in the x-raydiffraction pattern. If yttrium is present, at least 70 percent of thetotal peak area of the x-ray diffraction scan is attributed to a cubicstructure, tetragonal structure, or a combination thereof with thebalance being monoclinic. For example, at least 75 percent, at least 80percent, or at least 85 percent of the total peak area of some x-raydiffraction scans can be attributed to a cubic crystal structure,tetragonal crystal structure, or a combination thereof. Cubic andtetragonal crystal structures tend to promote the formation of lowaspect ratio primary particles having a cube-like shape when viewedunder an electron microscope.

Stated differently, at least 70 weight percent of the zirconiananoparticles are present in the cubic or tetragonal crystal structure.In some embodiments, at least 75 weight percent, at least 80 weightpercent, at least 85 weight percent, at least 90 weight percent, or atleast 95 weight percent of the zirconia nanoparticles are present in thecubic or tetragonal crystal structure.

The zirconia particles usually have an average primary particle size nogreater than 50 nanometers, no greater than 40 nanometers, no greaterthan 30 nanometer, no greater than 25 nanometers, no greater than 20nanometers, no greater than 15 nanometers, or no greater than 10nanometers. The primary particle size, which refers to thenon-associated particle size of the zirconia particles, can bedetermined by x-ray diffraction as described in the Examples section.

The effluent of the hydrothermal treatment usually containsnon-associated zirconia-containing nanoparticles. Thezirconia-containing sol effluent is typically clear. In contrast,zirconia-containing sols that contain agglomerated or aggregatedparticles usually tend to have a milky or cloudy appearance. Thezirconia-containing sols often have a high optical transmission due tothe small size and non-associated form of the primary zirconia particlesin the sol. High optical transmission of the sol can be desirable in thepreparation of transparent or translucent composite materials. As usedherein, “optical transmission” refers to the amount of light that passesthrough a sample (e.g., a zirconia-containing sol) divided by the totalamount of light incident upon the sample. The percent opticaltransmission may be calculated using the equation100(I/I _(O))where I is the light intensity passing though the sample and I_(O) isthe light intensity incident on the sample. The optical transmission maybe determined using an ultraviolet/visible spectrophotometer set at awavelength of 600 nanometers with a 1 centimeter path length. Theoptical transmission is a function of the amount of zirconia in a sol.For zirconia-containing sols having about 1 weight percent zirconia, theoptical transmission is typically at least 70 percent, at least 80percent, or at least 90 percent. For zirconia-containing sols havingabout 10 weight percent zirconia, the optical transmission is typicallyat least 20 percent, at least 50 percent, or at least 70 percent.

The extent of association between the primary particles can bedetermined from the hydrodynamic particle size. The hydrodynamicparticle size is measured using Photon Correlation Spectroscopy and isdescribed in more detail in the Examples section below. The term“hydrodynamic particle size” and “volume-average particle size” are usedinterchangeably herein. If the particles of zirconia are associated, thehydrodynamic particle size provides a measure of the size of theaggregates and/or agglomerates of primary particles in the zirconia sol.If the particles of zirconia are non-associated, the hydrodynamicparticle size provides a measure of the size of the primary particles.

A quantitative measure of the degree of association between the primaryparticles in the zirconia sol is the dispersion index. As used hereinthe “dispersion index” is defined as the hydrodynamic particle sizedivided by the primary particle size. The primary particle size (e.g.,the weighted average crystallite size) is determined using x-raydiffraction techniques and the hydrodynamic particle size (e.g., thevolume-average particle size) is determined using Photon CorrelationSpectroscopy. As the association between primary particles in the soldecreases, the dispersion index approaches a value of 1 but can besomewhat higher or lower. The zirconia-containing nanoparticlestypically have a dispersion index of about 1 to 5, about 1 to 4, about 1to 3, about 1 to 2.5, or about 1 to 2.

Photon Correlation Spectroscopy can be used to further characterize thezirconia particles in the sol. For example, the intensity of the lightscattered by particles is proportional to the sixth power of theparticle diameter. Consequently, a light-intensity distribution tends tobe more sensitive to larger particles than smaller ones. One type ofintensity-based size available from Photo Correlation Spectroscopy isthe Z-average size. It is calculated from the fluctuations in theintensity of scattered light using a cumulants analysis. This analysisalso provides a value called the polydispersity index, which is ameasure of the breadth of the particle size distribution. Thecalculations for the Z-average size and polydispersity index are definedin the ISO standard document 13321:1996 E.

The zirconia particles tend to have a Z-average size that is no greaterthan 70 nanometers, no greater than 60 nanometers, no greater than 50nanometers, no greater than 40 nanometers, no greater than 35nanometers, or no greater than 30 nanometers.

The polydispersity index is often less than 0.5, less than 0.4, lessthan 0.3, less than 0.2, or less than 0.1. A polydispersity index near0.5 often indicates a broad particle size distribution while apolydispersity index near 0.1 often indicates a narrow particle sizedistribution.

In addition to the Z-average size and polydispersity index, a completelight-intensity distribution can be obtained during analysis usingPhoton Correlation Spectroscopy. This can further be combined with therefractive indices of the particles and the refractive index of thesuspending medium to calculate a volume distribution for sphericalparticles. The volume distribution gives the percentage of the totalvolume of particles corresponding to particles of a given size range.The volume-average size is the size of a particle that corresponds tothe mean of the volume distribution. Since the volume of a particle isproportional to the third power of the diameter, this distribution isless sensitive to larger particles than an intensity-based sizedistribution. That is, the volume-average size will typically be asmaller value than the Z-average size. The zirconia sols typically havea volume-average size that is no greater than 50 nanometers, no greaterthan 40 nanometers, no greater than 30 nanometers, no greater than 25nanometers, no greater than 20 nanometers, or no greater than 15nanometers. The volume-average size is used in the calculation of thedispersion index.

In another aspect, a method of preparing a composite material isprovided. The method includes preparing non-associated,zirconia-containing nanoparticles as described above and then suspendingor dispersing the zirconia-containing nanoparticles in an organicmatrix. Any suitable means known in the art can be used to suspend ordisperse the zirconia-containing nanoparticles in an organic matrix.Preferably, any method used for suspending or dispersing thezirconia-containing nanoparticles in the organic matrix material doesnot result in agglomeration or aggregation.

In some applications, the zirconia-containing nanoparticles can besuspended or dispersed in the organic matrix without any further surfacemodification. The organic matrix can be added directly to the effluentfrom the continuous hydrothermal reactor. Alternatively, the organicmatrix can be added to the effluent after treatment to remove at leastsome of the water, after treatment to remove at least some of thecarboxylic acids and/or anions thereof, or after both treatments. Theorganic matrix that is added is often a polymerizable composition thatis subsequently polymerized and/or crosslinked to form a polymericmaterial.

In one example, the effluent of the continuous hydrothermal reactor canbe subjected to a solvent exchange process. An organic solvent having ahigher boiling point than water is added to the effluent. The water thencan be removed by a method such as, for example, distillation, rotaryevaporation, oven drying, or the like. Depending on the conditions usedfor removing the water, at least a portion of the dissolved carboxylicacid and/or anion thereof can also be removed. The organic matrix can beadded to the treated effluent (i.e., the organic matrix is added to thezirconia-containing nanoparticle suspended in the organic solvent usedin the solvent exchange process).

Alternatively, an organic matrix having a higher boiling point thanwater can be added to the effluent along with an optional solvent. Thewater and optional solvent can be removed by a method such as, forexample, distillation, rotary evaporation, oven drying, or the like.Depending on the conditions used for removing the water and optionalsolvent, at least a portion of the dissolved carboxylic acid and/oranion thereof can also be removed.

In another example, the dissolved carboxylic acid and/or anion thereofin the effluent from the continuous hydrothermal reactor can be removedby a process such as dialysis, diafiltration, or ion exchange. Anorganic matrix and optional organic solvent can be added either beforeor after removal of most of the water from the treated effluent (e.g.,the effluent treated by dialysis, diafiltration, or ion exchange). Ifthe organic matrix and optional organic solvent is added before removalof most of the water, the boiling point of the organic matrix isselected to be greater than the boiling point of water. The water can beremoved using a method such as distillation, rotary evaporation, or ovendrying. The optional organic solvent is typically removed with thewater. Alternatively, the pH of the treated effluent can be adjusted toprecipitate the zirconia-containing nanoparticles from the treatedeffluent. The precipitated zirconia-containing nanoparticles can becollected by filtration or centrifugation. Any remaining water can beremoved either before or after mixing the filtered or centrifugedzirconia-containing nanoparticle with an organic matrix.

In yet another example, the zirconia-sol can be dried to form a powder.The dried powder can be suspended or dispersed in water or a solvent.Alternatively, the dried powder can be suspended or dispersed in anorganic matrix with or without the addition of an optional solvent.

In other applications, however, the zirconia-containing nanoparticlesare further treated with a surface modification agent to further improvecompatibility with the organic matrix material. Surface modificationagents may be represented by the formula A-B where the A group iscapable of attaching to the surface of a zirconia-containingnanoparticle and B is a compatibility group. Group A can be attached tothe surface by adsorption, formation of an ionic bond, formation of acovalent bond, or a combination thereof. Group B can be reactive ornon-reactive and often tends to impart characteristics to thezirconia-containing nanoparticles that are compatible (i.e., miscible)with an organic solvent, with an organic matrix material, or both. Forexample, if the solvent is non-polar, group B is typically selected tobe non-polar as well. Suitable B groups include linear or branchedhydrocarbons that are aromatic, aliphatic, or both aromatic andaliphatic. The surface modifying agents include, but are not limited to,carboxylic acids and/or anions thereof, sulfonic acids and/or anionsthereof, phosphoric acids and/or anions thereof, phosphonic acids and/oranions thereof, silanes, amines, and alcohols.

In some embodiments, the surface modification agent is a carboxylic acidand/or anion thereof and the compatibility B group can impart a polarcharacter to the zirconia-containing nanoparticles. For example, thesurface modification agent can be a carboxylic acid and/or anion thereofhaving a polyalkylene oxide group. In some embodiments, the carboxylicacid surface modification agent is of the following formula.H₃C—[O—(CH₂)_(y)]_(x)-Q-COOHIn this formula, Q is a divalent organic linking group, x is an integerin the range of 1 to 10, and y is an integer in the range of 1 to 4. Thegroup Q is often an alkylene group, alkenylene group, arylene, oxy,thio, carbonyloxy, carbonylimino, or a combination thereof.Representative examples of this formula include, but are not limited to,2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEAA) and2-(2-methoxyethoxy)acetic acid (MEAA). Other representative examples arethe reaction product of an aliphatic or aromatic anhydride and apolyalkylene oxide mono-ether such as succinic acidmono-[2-(2-methoxy-ethoxy)-ethyl]ester, maleic acidmono-[2-(2-methoxy-ethoxy)-ethyl]ester, and glutaric acidmono-[2-(2-methoxy-ethoxy)-ethyl]ester.

Still other carboxylic acid surface modifying agents are the reactionproduct of phthalic anhydride with an organic compound having a hydroxylgroup. Suitable examples include, for example, phthalic acidmono-(2-phenylsulfanyl-ethyl)ester, phthalic acidmono-(2-phenoxy-ethyl)ester, or phthalic acidmono-[2-(2-methoxy-ethoxy)-ethyl]ester. In some examples, the organiccompound having a hydroxyl group is a hydroxyl alkyl(meth)acrylate suchas hydroxyethyl(meth)acrylate, hydroxypropyl(meth)acrylate, orhydroxylbutyl(meth)acrylate. Examples include, but are not limited to,succinic acid mono-(2-acryloyloxy-ethyl)ester, maleic acidmono-(2-acryloyloxy-ethyl)ester, glutaric acidmono-(2-acryloyloxy-ethyl)ester, phthalic acidmono-(2-acryloyloxy-ethyl)ester, and phthalic acidmono-(2-acryloyl-butyl)ester. Still others include mono-(meth)acryloxypolyethylene glycol succinate and the analogous materials made frommaleic anhydride glutaric anhydride, and phthalic anhydride.

In other examples, the surface modification agent is the reactionproduct of polycaprolactone and succinic anhydride.

In other embodiments, the surface modification agent is a carboxylicacid and/or anion thereof and the compatibility B group can impart anon-polar character to the zirconia-containing nanoparticles. Forexample, the surface modification agent can be a carboxylic acid and/oranion thereof having a linear or branched aromatic group or aliphatichydrocarbon group. Representative examples of include octanoic acid,dodecanoic acid, stearic acid, oleic acid, and combinations thereof.

In still other embodiments, the surface modification agent is acarboxylic acid and/or anion thereof and the compatibility B group canbe reactive with a polymerizable organic matrix (e.g., the B groupcontains a polymerizable group). Reactive carboxylic acid surfacemodifying agents (e.g., carboxylic acids with polymerizable groups)include, for example, acrylic acid, methacrylic acid, beta-carboxyethylacrylate, mono-2-(methacryloxyethyl)succinate, and combinations thereof.A useful surface modification agent that can impart both polar characterand reactivity to the zirconia-containing nanoparticles ismono(methacryloxypolyethyleneglycol)succinate. This material may beparticularly suitable for addition to radiation curable acrylate and/ormethacrylate organic matrix materials.

Exemplary silanes include, but are not limited to, alkyltrialkoxysilanessuch as n-octyltrimethoxysilane, n-octyltriethoxysilane,isooctyltrimethoxysilane, dodecyltrimethoxysilane,octadecyltrimethoxysilane, propyltrimethoxysilane, andhexyltrimethoxysilane; methacryloxyalkyltrialkoxysilanes oracryloxyalkyltrialkoxysilanes such as3-methacryloxypropyltrimethoxysilane, 3-acryloxypropyltrimethoxysilane,and 3-(methacryloxy)propyltriethoxysilane;methacryloxyalkylalkyldialkoxysilanes oracryloxyalkylalkyldialkoxysilanes such as3-(methacryloxy)propylmethyldimethoxysilane, and3-(acryloxypropyl)methyldimethoxysilane;methacryloxyalkyldialkylalkoxysilanes oracyrloxyalkyldialkylalkoxysilanes such as3-(methacryloxy)propyldimethylethoxysilane;mercaptoalkyltrialkoxylsilanes such as 3-mercaptopropyltrimethoxysilane;aryltrialkoxysilanes such as styrylethyltrimethoxysilane,phenyltrimethoxysilane, phenyltriethoxysilane, andp-tolyltriethoxysilane; vinyl silanes such asvinylmethyldiacetoxysilane, vinyldimethylethoxysilane,vinylmethyldiethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane,vinyltriacetoxysilane, vinyltriisopropoxysilane, vinyltrimethoxysilane,vinyltriphenoxysilane, vinyltri-t-butoxysilane,vinyltris(isobutoxy)silane, vinyltriisopropenoxysilane, andvinyltris(2-methoxyethoxy)silane; 3-glycidoxypropyltrialkoxysilane suchas glycidoxypropyltrimethoxysilane; polyether silanes such asN-(3-triethoxysilylpropyl)methoxyethoxyethoxyethyl carbamate (PEG3TES),N-(3-triethoxysilylpropyl)methoxyethoxyethoxyethyl carbamate (PEG2TES),and SILQUEST A-1230; and combinations thereof.

Any known method of adding the surface modification agent to thezirconia-containing nanoparticles can be used. The surface modificationagent can be added before or after any removal of at least a portion ofthe carboxylic acids and/or anions thereof from the zirconia-containingsol. The surface modification agent can be added before or after removalof the water from the zirconia-containing sol. The organic matrix can beadded after surface modification or simultaneously with surfacemodification.

In one exemplary method, the effluent of the continuous hydrothermalreactor can be subjected to a solvent exchange process. An organicsolvent having a boiling point greater than water is added to theeffluent. The water can be removed by a method such as, for example,distillation, rotary evaporation, oven drying, or the like. Depending onthe conditions used for removing the water, at least a portion of thedissolved carboxylic acid and/or anion thereof can also be removed. Thesurface modification agent can be added either before or after the waterremoval step. The surface modification agent can be selected tofacilitate the extraction of the zirconia-containing nanoparticles intothe organic solvent used in the solvent exchange process. The organicmatrix is typically added after removal of the water. The organicsolvent used in the solvent exchange process can often be removed afteraddition of the organic matrix using a method such as distillation,rotary evaporation, oven drying, or the like. Alternatively, the organicmatrix, organic solvent, and the additional surface modification agentcan be added at the same time to the effluent of the continuoushydrothermal reactor.

In another example, the effluent of the continuous hydrothermal reactorcan be dried to form a powder. The dried powder can be suspended ordispersed in an organic solvent or water to which a surface modificationagent has been added. The surface modification agent is selected tofacilitate the dispersion of the zirconia-containing nanoparticles intothe liquid medium.

In an alternative example, the effluent of the continuous hydrothermalreactor can be treated with a surface modification agent before beingdried to form a powder. The surface modification agent is added to theeffluent of the continuous hydrothermal reactor. The surfacemodification agent is selected to facilitate the dispersion of thezirconia-containing nanoparticles into an organic matrix. The treatedeffluent is then dried to a powder. The dried powder can be suspended ordispersed in an organic matrix.

In some embodiments, the effluent from the hydrothermal reactor isconcentrated (but not dried to a powder) to remove at least a portion ofthe aqueous based medium. This concentration process often removes atleast a portion of the carboxylic acids and/or anions thereof.Optionally, additional dissolved carboxylic acid and/or anion thereof inthe concentrate can be removed by a treatment such as dialysis,diafiltration, or ion exchange. The concentrated and optionally treatedzirconia-containing sol can be mixed with the surface modification agentand an optional organic solvent. After surface treatment, the surfacemodified zirconia-containing nanoparticle can be mixed with an organicmatrix. The optional organic solvent and the remaining water can beremoved before or after addition of the organic matrix. Alternatively,the concentrated zirconia-containing sol can be mixed with both theorganic matrix and the surface modification agent in the presence of anoptional organic solvent. The optional organic solvent and the remainingwater can be removed after surface modification.

In another embodiment, the effluent from the hydrothermal reactor isconcentrated (but not dried to a powder) to remove at least a portion ofthe aqueous based medium. This concentration process often removes atleast a portion of the carboxylic acids and/or anions thereof.Optionally, additional dissolved carboxylic acid and/or anion thereof inthe concentrate can be removed by a treatment such as dialysis,diafiltration, or ion exchange. The concentrated and optionally treatedzirconia-containing sol can be mixed with the surface modificationagent. After surface treatment, the mixture can be dried to form apowder. This surface modified zirconia-containing nanoparticle powdercan be dispersed into an organic matrix.

In yet another example, the dissolved carboxylic acid and/or anionthereof in the effluent from the continuous hydrothermal reactor can beremoved by a process such as dialysis, diafiltration, or ion exchange.The surface modification agent can be added directly to the treatedeffluent (e.g., the effluent treated using ion exchange, diafiltration,or dialysis). Optionally, a polar co-solvent may be added to increasethe solubility of the surface modification agent in the aqueous phase.Suitable polar co-solvents include water-miscible organic compounds suchas, for example, 1-methoxy-2-propanol, ethanol, isopropanol, ethyleneglycol, N,N-dimethylacetamide, N-methylpyrrolidone, or the like. Theorganic matrix can be added either before or after removal of most ofthe water and optional polar co-solvents. The water and optional polarco-solvent can be removed, for example, by distillation, rotaryevaporation, or oven drying if the organic matrix has a higher boilingpoint. The water and optional polar co-solvent can also be removed byprecipitation of the surface modified zirconia-containing nanoparticleby adjusting the pH. Alternatively, the surface modification agent canchange the polarity of the zirconia-containing nanoparticles resultingin precipitation of the surface modified zirconia-containingnanoparticles. The precipitated zirconia-containing nanoparticle can beseparated from the liquid phase by filtration or centrifugation. Anyremaining water and optional co-solvent can be removed either before orafter mixing the filtered or centrifuged zirconia-containingnanoparticles with an organic matrix.

The surface modification reactions can occur at room temperature (e.g.,20° C. to 25° C.) or at an elevated temperature (e.g., up to about 95°C.). When the surface modification agents are acids such as carboxylicacids, the zirconia-containing nanoparticles typically can besurface-modified at room temperature. When the surface modificationagents are silanes, the zirconia-containing nanoparticles are typicallysurface modified at elevated temperatures.

The organic matrix typically includes a polymeric material or aprecursor to a polymeric material such as a monomer or an oligomerhaving a polymerizable group. Any suitable technique can be used tocombine the zirconia-containing nanoparticles with the organic matrix.For example, if the organic matrix is a precursor to a polymericmaterial, the zirconia-containing nanoparticles can be added prior tothe polymerization reaction. If the polymeric material is athermoplastic, the polymeric material and the zirconia-containingnanoparticles can be combined using a process such as extrusion,milling, or Brabender mixing. The composite material containing aprecursor of a polymeric material is often shaped or coated beforepolymerization.

Representative examples of monomers include, but are not limited to,(meth)acrylates, styrenes, epoxies, and the like. Representativeexamples of reactive oligomers include, but are not limited to,(meth)acrylated polyesters, (meth)acrylated polyurethanes, or acrylics.Representative examples of polymeric material include, but are notlimited to, polyolefins, polyesters, polyurethanes, poly(meth)acrylates,polystyrenes, polycarbonates, and polyimides.

One exemplary process for suspending or dispersing thezirconia-containing nanoparticles in an organic matrix includesconcentrating the effluent from the hydrothermal reactor to about 40percent solids using a method such as distillation or rotaryevaporation. A co-solvent and surface modification agent is then addedto the concentrate. After addition of the organic matrix, theco-solvent, water, and at least a portion of the dissolved carboxylicacid and/or anion thereof are removed. In a more specific example, thesurface modification agent is a carboxylic acid such as a carboxylicacid having a polyalkelyene oxide group and the organic matrix is thereaction product of at least one (meth)acrylate.

In some more specific methods of suspending or dispersing thezirconia-containing nanoparticles in an organic matrix, thezirconia-containing nanoparticles are treated with a silane surfacemodifying agent. Prior to addition of the silane surface modifying agentor agents, the carboxylic acid and/or anion thereof in the sol hastypically been reduced or removed by a method such as ion exchange,dialysis, diafiltration, or concentration (drying) followed by dilutionwith water. The silane surface modifying agent is combined with thezirconia-containing nanoparticles in the form of a sol. In some methods,two or more silane surface modifying agents are added. Thezirconia-containing sol often has a pH in the range of 2 to 5 and anoptional miscible, organic solvent can be present. The resulting mixtureis often heated for 3 to 16 hours at 80° C. to 90° C. but other timesand temperatures can be used.

After cooling, the mixture is added to a dilute aqueous ammoniasolution. Other base materials may be used as an alternative to theammonia solution. The addition of the mixture to the base will typicallylead to precipitation of the zirconia-containing nanoparticles. The baseis believed to facilitate removal of the attached (e.g., sorbed)carboxylic acids and/or anions thereof from the surface of thesilane-treated zirconia-containing nanoparticles. Subsequent filtrationand washing of the solids allows for further removal of the acids and/oranions thereof. Following filtration, the silane-treatedzirconia-containing nanoparticles can be dispersed in a solvent andsubsequently incorporated into a resin via solvent exchange.Alternatively, the solids from filtration can be dried to a powder andthen dispersed or suspended in the resin.

EXAMPLES

These examples are for illustrative purposes only and are not meant tobe limiting on the scope of the appended claims. All parts, percentages,ratios, etc. in the examples and the rest of the specification are byweight, unless noted otherwise. Solvents and other reagents used wereobtained from Sigma-Aldrich Chemical Company; Milwaukee, Wis. unlessotherwise noted.

Table of Abbreviations Abbreviation or Trade Designation DescriptionMEEAA 2-(2-(2-methoxyethoxy) ethoxy) acetic acid Zirconium An aqueoussolution of zirconium acetate containing acetate nominally 16.3 weightpercent Zr that is commercially (Solution A) available from MagnesiumElektron, Inc. (Flemington, NJ). Zirconium An aqueous solution ofzirconium acetate containing acetate nominally 16.3 weight percent Zrthat is commercially (Solution B) available from Magnesium Elektron,Inc. (Flemington, NJ). Zirconium An aqueous solution of zirconiumacetate containing acetate nominally 16.3 weight percent Zr that iscommercially (Solution C) available from Magnesium Elektron, Inc.(Flemington, NJ). DI water De-ionized water Yttrium Y(CH₃COO)₃•4H₂O,which is commercially available acetate from AMR Technologies Inc(Toronto Canada) hydrate TBPEA Tribromophenoxyethyl acrylate, which iscommercially available from Dai-Chi Kogyo Seiyaku Co., Ltd. (Kyoto,Japan) PEA Phenoxyethyl acrylate, which is commercially available fromSartomer (Exton, PA) PROSTABB A radical inhibitor that is commerciallyavailable from 5198 Ciba Specialties (Hawthorne, NY) 1-Methoxy-2- Analcohol that is commercially available from Aldrich propanol Chemical(Milwaukee, WI) Maleic acid An organic acid that is commerciallyavailable from Aldrich Chemical TMPTA Trimethylolpropane triacrylatethat is commercially available from Sartomer Company Inc. (Exton PA)HEAP Phthalic acid mono-ester of 2-hydroxylethyl acrylate as describedin Preparatory Example 1 NaCl Sodium chloride, which is commerciallyavailable from EM Science (Gibbstown, NJ) CaCl₂ Calcium chloride, whichis commercially available from EM Science (Gibbstown, NJ) AMBERLITE Ionexchange resin that is available from Aldrich IR-120 Chemical Company(Milwaukee, WI) or Rohm and Haas (Philadelphia, PA) Phthalic Ananhydride that is commercially available from anhydride Aldrich ChemicalCompany (Milwaukee, WI) 2-Hydroxyethyl An acrylate monomer that iscommercially available acrylate from Aldrich Chemical Company(Milwaukee, WI) 2-Hydroxyethyl A methacrylate monomer that iscommercially available methacrylate from Aldrich Chemical Company(Milwaukee, WI) SR603 Polyethylene Glycol (400) dimethacrylate that iscommercially available from Sartomer Company Inc. (Exton, PA)Methacryloxy A silane coupling agent that is commercially availablepropyltri- from Aldrich Chemical Company (Milwaukee, WI) methoxy silaneSILQUEST A silane coupling agent that is commercially available A1230from GE Silicones-OSi Specialties (Wilton, CT) Triethyl- A base that iscommercially available from Aldrich amine Chemical Company (Milwaukee,WI) Zirconium A zirconium salt that is commercially available fromoxynitrate Teledyne Wah Chang Albany (Albany, OR) Zirconium A zirconiumsalt that is commercially available from dichloride Avocado ResearchChemicals Ltd. (Lancashire, UK) oxide octahydrate Resin 1 A mixturecontaining 50/30/20 weight percent TBPEA/ PEA/TMPTA Ex Example CExComparative ExampleTest MethodsPhoton Correlation Spectroscopy (PCS)

Particle size measurements were made using a Zeta Sizer—Nano Series,Model ZEN3600 instrument equipped with a red laser having a 633nanometer wavelength. This instrument is commercially available fromMalvern Instruments Inc. (Westborough, Mass.). The samples, which wererun as prepared, were poured into a disposable, 1 centimeter squarepolystyrene cuvette to a liquid depth of 10 to 15 mm. The cuvette wasplaced into the instrument and equilibrated at 25° C. The instrumentparameters were set as follows: dispersant refractive index 1.330,dispersant viscosity 0.8872 mPa-second, material refractive index 2.10,and material absorption value 0.10 units. The automatic size-measurementprocedure was then run. The instrument automatically adjusted thelaser-beam position and attenuator setting to obtain the bestmeasurement of particle size.

The ZEN3600 instrument illuminated the sample with a laser and analyzedthe intensity fluctuations of the light scattered from the particles atan angle of 173 degrees. The method of Photon Correlation Spectroscopy(PCS) was used by the instrument to calculate the particle size. PCSuses the fluctuating light intensity to measure the Brownian motion ofparticles in the liquid. The particle size is then calculated to be thediameter of a sphere that moves at the measured speed.

The intensity of the light scattered by a particle is proportion to thesixth power of the particle diameter. The Z-average size or cumulantmean is a mean calculated from the intensity fluctuations and thecalculation is based on the assumptions that the particles aremono-modal, mono-disperse, and spherical. Related functions calculatedfrom the fluctuating light intensity are the Intensity Distribution andits mean. The mean of the Intensity Distribution is calculated based onthe assumption that the particles are spherical. Both the Z-average sizeand the Intensity Distribution mean are more sensitive to largerparticles than smaller ones.

The Volume Distribution gives the percentage of the total volume ofparticles corresponding to particles of a given size range. Thevolume-average size is the size of a particle that corresponds to themean of the Volume Distribution. Since the volume of a particle isproportional to the third power of the diameter, this distribution isless sensitive to larger particles than the Z-average size. Thus, thevolume-average size will typically be a smaller value than the Z-averagesize.

Crystalline Structure and Size (XRD Analysis)

Dried zirconia samples were ground by hand using an agate mortar andpestle. A liberal amount of the sample was applied by spatula to a glassmicroscope slide on which a section of double sided adhesive tape hadbeen adhered. The sample was pressed into the adhesive on the tape byforcing the sample against the adhesive with the spatula blade. Excesssample was removed by scraping the sample area with the edge of thespatula blade, leaving a thin layer of particles adhered to theadhesive. Loosely adhered materials remaining after the scraping wereremoved by forcefully tapping the microscope slide against a hardsurface. In a similar manner, corundum (Linde 1.0 μm alumina polishingpowder, Lot Number C062, Union Carbide, Indianapolis, Ind.) was preparedand used to calibrate the X-ray diffractometer for instrumentalbroadening.

X-ray diffraction scans were obtained using a Philips verticaldiffractometer having a reflection geometry, copper K_(α) radiation, anda proportional detector registry of the scattered radiation. Thediffractometer was fitted with variable incident beam slits, fixeddiffracted beam slits, and a graphite diffracted beam monochromator. Thesurvey scan was recorded from 25 to 55 degrees two theta (2θ) using astep size of 0.04 degrees and a dwell time of 8 seconds. X-ray generatorsettings of 45 kV and 35 mA were used. Data for the corundum standardwas collected on three separate areas of several individual corundummounts. Likewise, data was collected on three separate areas of the thinlayer sample mount.

The observed diffraction peaks were identified by comparison toreference diffraction patterns contained within the International Centerfor Diffraction Data (ICDD) powder diffraction database (sets 1-47,ICDD, Newton Square, Pa.). The diffraction peaks for the samples wereattributed to either cubic/tetragonal (C/T) or monoclinic (M) forms ofzirconia. The (1 1 1) peak for the cubic phase and (1 0 1) peak for thetetragonal phase could not be separated so these phases were reportedtogether. The amounts of each zirconia form were evaluated on a relativebasis and the form of zirconia having the most intense diffraction peakwas assigned the relative intensity value of 100. The strongest line ofthe remaining crystalline zirconia form was scaled relative to the mostintense line and given a value between 1 and 100.

Peak widths for the observed diffraction maxima due to corundum weremeasured by profile fitting. The relationship between mean corundum peakwidths and corundum peak position (2θ) was determined by fitting apolynomial to these data to produce a continuous function used toevaluate the instrumental breadth at any peak position within thecorundum testing range. Peak widths for the observed diffraction maximadue to zirconia were measured by profile fitting the observeddiffraction peaks. The following peak widths were evaluated depending onthe zirconia phase found to be present:

-   -   Cubic/Tetragonal (C/T): (1 1 1)    -   Monoclinic (M): (−1 1 1), and (1 1 1)

A Pearson VII peak shape model with K_(α1) and K_(α2) wavelengthcomponents and linear background model were used for all measurements.Widths were calculated as the peak full width at half maximum (FWHM)having units of degrees. The profile fitting was accomplished by use ofthe capabilities of the JADE diffraction software suite. Sample peakwidths were evaluated for the three separate data collections obtainedfor the same thin layer sample mount.

Sample peaks were corrected for instrumental broadening by interpolationof instrumental breadth values from corundum instrument calibration andcorrected peak widths converted to units of radians. The Scherrerequation was used to calculate the primary crystal size.Crystallite Size(D)=Kλ/β(cos θ)In the Scherrer equation, K is the form factor (here 0.9), λ is thewavelength (1.540598 Å), β is the calculated peak width after correctionfor instrumental broadening (in radians), and θ equals half the peakposition (scattering angle). β is equal to [calculated peakFWHM−instrumental breadth] (converted to radians) where FWHM is fullwidth at half maximum. The cubic/tetragonal (C/T) mean crystallite sizewas measured as the average of three measurements using (1 1 1) peak.That is,C/T mean crystallite size=[D(1 1 1)_(area 1) +D(1 1 1)_(area 2) +D(1 11)_(area 3)]/3.The monoclinic (M) crystallite size was measured as the average of threemeasurement using the (−1 1 1) peak and three measurements using the (11 1) peak.M mean crystallite size=[D(−1 1 1)_(area 1) +D(−1 1 1)_(area 2) +D(−1 11)_(area 3) +D(1 1 1)_(area 1) +D(1 1 1)_(area 2) +D(1 1 1)_(area 3)]/6The weighted average of the cubic/tetragonal (C/T) and monoclinincphases (M) were calculated.Weighted average=[(% C/T)(C/T size)+(% M)(M size)]/100In this equation, % C/T equals the percent crystallinity contributed bythe cubic and tetragonal crystallite content of the ZrO₂ particles; C/Tsize equals the size of the cubic and tetragonal crystallites; % Mequals the percent crystallinity contributed by the monocliniccrystallite content of the ZrO₂ particles; and M size equals the size ofthe monoclinic crystallites.Dispersion Index

The dispersion index is equal to the volume-average size measured usingPhoton Correlation Spectroscopy divided by the weighted averagecrystallite size measured by XRD.

Polydispersity Index

The polydispersity index is a measure of the breadth of the particlesize distribution and is calculated along with the Z-average size in thecumulants analysis of the intensity distribution using PhotonCorrelation Spectroscopy. For values of the polydispersity index of 0.1and below, the breadth of the distribution is considered narrow. Forvalues above 0.5, the breadth of the distribution is considered broadand it is unwise to rely on the Z-average size to fully characterize theparticle size. Instead, one should characterize the particles using adistribution analysis such as the intensity or volume distribution. Thecalculations for the Z-average size and polydispersity index are definedin the ISO standard document 13321:1996 E.

Weight Percent Solids

The weight percent solids were determined by drying a sample weighing 3to 6 grams at 120° C. for 30 minutes. The weight percent solids can becalculated from the weight of the wet sample (i.e., weight beforedrying, weight_(wet)) and the weight of the dry sample (i.e., weightafter drying, weight_(dry)) using the following equation.Wt-% solids=100(weight_(dry))/weight_(wet)Thermal Gravimetric Analysis (TGA)

The percent conversion of the zirconium-containing intermediate and theweight percent inorganic oxides were determined by thermal gravimetricanalysis using a Model 2950 TGA from TA Instruments (New Castle, Del.).

The percent conversion of the zirconium-containing sample under analysisis given by the following equation% Conversion=100(A−B)/(A−C)where A is the percent weight loss of the feedstock, B is the percentweight loss of the zirconium-containing sample under analysis, and C isthe percent weight loss of a zirconia-containing standard known orbelieved to be completely converted.

To determine the percent weight loss, a sample of the feedstock, asample of the zirconia-containing sample under analysis and thezirconia-containing standard were each dried at 120° C. in an oven for30 minutes analysis. Each sample was in the range of 3 to 6 grams. Eachdried sample (e.g., 30 to 60 milligrams) was equilibrated at 85° C. inthe TGA. The temperature was then increased at a rate of 20° C./minuteto 200° C., held at 200° C. for 20 minutes, increased at a rate of 20°C./minute to 900° C., and held at 900° C. for 20 minutes. The organicmaterial was volatilized between 200° C. and 900° C. leaving only theinorganic oxides such as ZrO₂ and Y₂O₃. The percent weight loss wascalculated using the following equation.% weight loss=100(%-weight_(200C)−%-weight_(900C))/%-weight_(900C)The %-weight_(200C) was calculated from the weight of each sample at200° C. (weight_(200C)) and from the weight of each dried sample(weight_(dry)) used for the analysis (e.g., sample dried at 120° C.before analysis).%-weight_(200C)=100(weight_(200C))/weight_(dry)The %-weight_(900C) was calculated from the weight of each sample at900° C. (weight_(900C)) and from the weight of each dried sample(weight_(dry)) used for the analysis (e.g., sample dried at 120° C.before analysis).%-weight_(900C)=100(weight_(900C))/weight_(dry)The weight percent inorganic oxide was calculated from the weightpercent solids and the weight percent oxide at 900° C. That is, theweight percent inorganic oxide can be calculated using the followingequation.wt-% inorganic oxides=(wt-% solids)(%-weight_(900C))/100Index of Refraction

The refractive index was measured using an Abbe refractometercommercially available from Milton Roy Co. (Ivyland, Pa.).

Viscosity Measurement: Bubble Time Method

The sample was added to a Pyrex test tube (13×100 mm Dow Corning 980013) with a beaded rim. The tube was marked in two locations positioned 2inches (5.05 cm) apart. The first mark was 1.25 inches (3.725 cm) fromthe base of the tube and the second mark was 3.25 inches (8.255 cm) fromthe base of the tube. The sample was poured into the tube to a heightcorresponding to the second mark. The tube was then sealed with a cork(#3) and then with vinyl electric tape. The tube containing the samplewas place in a water bath set at 40° C. (104° F.) and equilibrated tothe temperature of the water bath. After equilibration, the tube wasremoved from the water bath and inverted to bring a bubble to the bottomof the tube. The tube was then invented a second time. The length oftime was measured using a stop watch for the bubble to pass between thefirst mark and the second mark. After the time was recorded, the tubewith the sample was placed back in the water bath to equilibrate. Themeasurement was repeated three times and the average was calculated.Longer times correspond to higher viscosities.

Inductively Coupled Plasma Atomic Emission Spectroscopy

Inductively Coupled Plasma Atomic Emission Spectroscopy was used toanalyze the feedstock and various other samples for the amount of sodiumand calcium. Liquid samples were aspirated into a high temperature argonplasma where desolvation, dissociation, atomization, and excitationoccur. Each element has a well established and characteristicwavelengths associated with emission from an excited state. Theintensity of the emission is typically proportional to the concentrationof the element. The concentration of the element can be calculated bycomparing the intensity of the emission with that of standards of knownconcentration.

The zirconium acetate solutions (0.20 to 0.3 grams) were accuratelyweighed into a centrifuge tube. Deionized water (40 mL) and hydrochloricacid (2 mL of EMD OMNITRACE concentrated hydrochloric acid (37-38percent)) was added. The solutions were then diluted to a total of 50 mLwith deionized water. Duplicates of each sample were prepared. Twoblanks containing just the hydrochloric acid and water were alsoprepared. The samples and blanks were analyzed on an ICP opticalemission spectrometer (Perkin Elmer Optima 4300 available from PerkinElmer, Shelton, Conn.). The instrument was calibrated using amulti-element standard containing at least sodium and calcium. Thestandards, which were obtained from solutions available from High PurityStandards, Charleston, S.C., had concentrations of 0.2 ppm, 0.5 ppm, and1.5 ppm (microgram per milliliter). The results were normalized to theamount of zirconium in the starting zirconium acetate solution.

Hydrothermal Reactor System

Hydrothermal Reactor A

This reactor was prepared from 100 ft (30 meters) of stainless-steeltubing (with an outside diameter of 0.25 inch (0.64 centimeters) and awall thickness of 0.035 inch (0.089 cm)) that was immersed in a bath ofpeanut oil heated to 206° C. Following the reactor tube, a coil of anadditional approximately 10 feet (3 meters) of stainless-steel tubingthat was immersed in an ice-water bath to cool the material. Thestainless-steel tubing has an outside diameter of 0.25 inch (0.64 cm)and a wall thickness of 0.035 inch (0.089 cm). A backpressure regulatorvalve was used to maintain an exit pressure of 400 psi.

Hydrothermal Reactor B

This reactor was prepared from 50 feet (15 meters) of Stainless SteelBraided Smooth Tube Hose (DuPont T62 Chemfluor PTFE, ¼ inch I.D., 0.040inch thick wall available from Saint-Gobain Performance Plastics,Beaverton, Mich.). That is, the inside of the reactor was PTFE. Thistube was immersed in a bath of peanut oil heated to 206° C. Followingthe reactor tube, a coil of an additional approximately 10 feet (3meters) of stainless-steel tubing that was immersed in an ice-water bathto cool the material. The stainless-steel tubing has an outside diameterof 0.25 inch (0.64 cm) and a wall thickness of 0.035 inch (0.089 cm). Abackpressure regulator valve was used to maintain an exit pressure of400 psi.

Preparative Example 1 Phthalic acid mono-(2-acryloyloxy-ethyl)ester(HEAP)

Phthalic anhydride (112.1 grams), 2-hydroxyethyl acrylate (87.9 grams),and triethylamine (0.44 grams) were mixed in a round bottom flask. Asmall amount of dry air was bubbled into the liquid reaction mixture.The reaction mixture was mixed and heated to 75° C. and held at thattemperature for six hours. The product was cooled to room temperature.NMR was used to confirm that the product was phthalic acidmono-(2-acryloyloxy-ethyl)ester. The product partially crystallized overtime. The product was mixed with 1-methoxy-2-propanol to obtain a 50weight percent solution.

Preparative Example 2 Master Batch 1

A master batch of Zirconium acetate and yttrium acetate was made bydissolving yttrium acetate (96.6 grams) in a zirconium acetate solution(solution A) (3500 grams). The master batch contained 37.4 weightpercent solids (16.61 weight percent Zr and 0.736 weight percent Y).

Example 1

Master batch 1 (1000 grams) and DI water (2000 grams) were mixed to givea 12 weight percent solids feedstock (5.33 weight percent Zr, 0.236weight percent Y). The feedstock was pumped at a rate of 18.6 mL/minthrough Hydrothermal Reactor A. A clear, crystalline-ZrO₂ containing solresulted (see Tables 1, 2, and 3). The XRD scan for Example 1 is shownin FIG. 3. The volume distribution was determined using PhotonCorrelation Spectroscopy and is shown in FIG. 2 (diamonds).

Example 2

Master batch 1 (777.8 grams) and DI water (2722 grams) were mixed togive a 8 weight percent solids feedstock (3.55 weight percent Zr, 0.157weight percent Y). The feedstock was pumped at a rate of 18.6 mL/minthrough Hydrothermal Reactor A. A slightly cloudy, crystalline-ZrO₂containing sol resulted (see Tables 1, 2, and 3).

Example 3

Zirconium acetate (solution A) (645.5 grams) and DI water (2354.5 grams)were mixed to form an 8 weight percent solids feedstock (3.55 weightpercent Zr, 0.0 weight percent Y). The feedstock was pumped at a rate of18.6 mL/min through Hydrothermal Reactor A. The slightly cloudy,crystalline-ZrO₂ containing sol resulted (see Tables 1, 2, and 3).

Example 4

Master batch 1 (1000 grams) and DI water (2000 grams) were mixed to givea 12 weight percent solids feedstock (5.33 weight percent Zr, 0.236weight percent Y). The feedstock was pumped at a rate of 18.6 mL/minthrough Hydrothermal Reactor A. A clear, crystalline-ZrO₂ containing solresulted (see Tables 1, 2, and 3).

Example 5

DI water (1294 grams) was mixed with zirconium acetate (solution A) (900grams). Yttrium acetate (24.2 grams) was added and the mixture stirredfor approximately 12 hours to prepare the 15 weight percent solidsfeedstock (6.66 weight percent Zr, 0.295 weight percent Y). Thefeedstock was pumped at a rate of 13.77 mL/min through HydrothermalReactor B. A clear, crystalline-ZrO₂ containing sol resulted (see Tables1, 2, and 3).

Example 6

DI water (1378.2 grams) was mixed with zirconium acetate (solution A)(1500 grams). Yttrium acetate (40.35 grams) was added and the mixturestirred for approximately 12 hours to prepare the 19 weight percentsolids feedstock (8.44 weight percent Zr, 0.374 weight percent Y). Thefeedstock was pumped at a rate of 13.77 mL/min through HydrothermalReactor B. A clear, crystalline-ZrO₂ containing sol resulted (see Tables1, 2, and 3).

Comparative Example 1

Master batch 1 (486 grams) and DI water (3014 grams) were mixed to givea 5 weight percent solids feedstock (2.22 weight percent Zr, 0.098weight percent Y). The solution was pumped at a rate of 18.6 mL/minthrough Hydrothermal Reactor A. A white crystalline-ZrO₂ containing solresulted (see Tables 1, 2, and 3). The large aggregate size alsoresulted in a settled precipitate. The volume distribution wasdetermined using Photon Correlation Spectroscopy and is shown in FIG. 2(squares).

Comparative Example 2

Master batch 1 (486 grams) and DI water (3014 grams) were mixed to givea 5 weight percent solids feedstock (2.22 weight percent Zr, 0.098weight percent Y). The solution was pumped at a rate of 14.9 mL/minthrough Hydrothermal Reactor A. A white crystalline-ZrO₂ containing solresulted (see Tables 1, 2, and 3). The large aggregate size alsoresulted in a settled precipitate.

Comparative Example 3

Master batch 1 (208.3 grams) and DI water (2791 grams) were mixed togive a 2.5 weight percent solids feedstock (1.11 weight percent Zr,0.049 weight percent Y). The solution was pumped at a rate of 18.6mL/min through Hydrothermal Reactor A. A cloudy, crystalline-ZrO₂containing sol resulted (see Tables 1, 2, and 3). The large aggregatesize also resulted in a settled precipitate.

TABLE 1 Effect of Feed Concentration on Product Appearance Resi- Gramsdence Yttrium Wt % Time Temp per gram Example Solids (min) (° C.)Zirconium Appearance Ex 1 12 28 206 0.044 Clear Ex 2 8 28 206 0.044Slightly cloudy Ex 3 8 28 206 0 Slightly cloudy Ex 4 12 28 206 0.044Clear Ex 5 15 35 206 0.044 Clear Ex 6 19 35 206 0.044 Clear CEx 1 5 28206 0.044 White, some settling CEx 2 5 35 206 0.044 White, some settlingCEx 3 2.5 28 206 0.044 Cloudy, some settling

TABLE 2 Effect of Feed Concentration on Product Particle Size Poly-Volume- Wt % Z-Average dispersity Average Example Solids size (nm) IndexSize (nm) Ex 1 12 15.0 0.377 5.45 Ex 2 8 27.9 7.09 Ex 3 8 27.4 0.2928.3  Ex 4 12 16.4 0.418 5.29 Ex 5 15 13.7 0.347 5.73 Ex 6 19 16.9 0.2866.72 CEx 1 5 135 Multimodal CEx 2 5 127 Multimodal CEx 3 2.5 221Multimodal

TABLE 3 Effect of Feed Concentration on Crystallite Size and DispersionIndex XRD M M C/T C/T % Average Dispersion Example Intensity size (nm)intensity size (nm) C/T Size (nm) index Ex 1 7 6 100 8.0 93 7.9 0.7 Ex 25 7.8 100 7.0 95 7.0 1.0 Ex 3 66 9.8 100 8.5 60 9.0 0.9 Ex 4 7 6.23 1006.5 93 6.5 0.8 Ex 5 9 7.3 100 8.7 92 8.6 0.6 Ex 6 20 5.7 100 8.9 83 8.40.8 CEx 1 5 9.8 100 9.5 95 9.5 >10 CEx 2 5 10.3 100 6.5 95 6.7 >10 CEx 33 7.5 100 5.0 97 5.1 >10

The data above show the unexpected result that hydrothermal treatment offeedstocks containing dissolved zirconium acetate resulted innon-aggregated ZrO₂ particles at high concentration but aggregation atlow concentration. The X-ray data (Table 3) confirm that allconcentrations resulted in zirconia crystals in the 6 to 10 nm range.All concentrations with yttrium (4 weight percent oxide) also resultedin over 80 percent of the zirconia being in the cubic/tetragonal phase.

The X-ray data show that both the examples and the comparative exampleshave similar crystallite sizes (primary particle size). The XRD scan forExample 1 is shown in FIG. 3. The degree of aggregation in the lowconcentration comparative examples was confirmed by the appearance ofthe sol (Table 1). The comparative examples were cloudy and white due tothe large aggregate size. Comparative Examples 1, 2 and 3 also have asignificant amount of solids settling, again due to large aggregatesize. Table 2 shows that the Z-average particle size is much higher forComparative Examples 1, 2, and 3 than the examples made at a highersolids concentration. The aggregation can be further seen in theparticle size distributions shown in FIG. 2. The volume averagedistribution clearly shows the aggregation for Comparative Examples 1(squares) compared to Example 1 (diamonds).

Example 7A to 7D

Zirconia-containing sols were made with different levels of sodium. Thefeedstock used in Example 5 was spiked with various amounts of NaCl. TheNaCl was dissolved with DI water to prepare a solution with 2.5 weightpercent solids. The amount of added 2.5 weight percent NaCl solution canbe found in Table 4. The sodium content of Example 5 was measured byInductively Coupled Plasma Atomic Emission Spectroscopy (ICP). Thesodium content of Examples 7A to 7D was calculated based on the amountof sodium added. The sodium content was expressed as the milligrams ofsodium divided by the grams of zirconium in each example. The ICPdetection limit is 0.023 milligram of sodium per gram of zirconium.

The feedstock for the hydrothermal reactions contained 15 weight percentsolids (6.66 weight percent Zr, 0.295 weight percent Y). Thezirconia-containing sols were made using a procedure similar to Example5, with the charges given in Table 4 below, at a temperature of 207° C.and using a 35 min residence time in Hydrothermal Reactor B.

TABLE 4 Reaction Conditions for Runs at Different Levels of Sodium IonZirconium Weight of milligrams acetate Yttrium DI NaCl solution sodium ÷solution acetate water added (2.5 grams Example A (grams) (grams)(grams) wt-%) (grams) zirconium Ex 5 900 24.2 1294 0 0.276 Ex 7A 90024.2 1294 4.65 0.583 Ex 7B 900 24.2 1294 9.3 0.889 Ex 7C 900 24.2 129418.6 1.503 Ex 7D 900 24.2 1294 27.9 2.116

The resulting zirconia-containing sols (summarized in Table 5) showedthat the particle size increased with the amount of sodium cation addedto the feedstock before heating. The clarity of the aqueous sol is alsoaffected by the addition of sodium cation to the feedstock. Lower sodiumlevels lead to higher clarity.

TABLE 5 Particle characterization milligrams Poly- Volume sodium ÷Z-Average dispersity Average Example grams zirconium Size (nm) IndexSize (nm) Ex 5 0.276 13.7 0.347 5.73 Ex 7A 0.583 13.9 0.344 6.81 Ex 7B0.889 15.3 0.280 7.72 Ex 7C 1.503 17.9 0.276 9.85 Ex 7D 2.116 20.8 0.27912.2

Example 8A to 8E

The zirconia-containing sols obtained in Examples 5 and 7 (Ex 5, Ex 7A,Ex 7B, Ex 7C, and Ex 7D) were concentrated to 40.5 weight percentzirconia via rotary evaporation (28.78 weight percent Zr, 1.27 weightpercent Y). The zirconia particles were surface treated and incorporatedinto a curable resin at 53 weight percent ZrO2 as described for Ex 8A to8E below.

Example 8A was prepared from Example 5. The zirconia-containing solconcentrated from Example 5 (50.01 grams, 41.89 weight percent ZrO2),MEEAA (2.46 grams), and 1-methoxy-2-propanol (35 grams) were chargedinto a 500 mL round-bottom flask in that order. HEAP from PreparativeExample 1 (4.26 grams at 50 weight percent HEAP in1-methoxy-2-propanol), maleic acid (0.067 grams), and1-methoxy-2-propanol (6.01 grams) were mixed together and heated todissolve the maleic acid. This solution was then charged to the aboveflask. PROSTABB 5198 (0.29 grams at 5 weight percent in DI water) andResin 1 (13.98 grams with a refractive index of 1.535) were then chargedto the flask. The volatiles were removed via rotary evaporation to yielda curable acrylate resin mixture containing approximately 53 weightpercent ZrO₂.

Samples 8B through 8E were made in the same manner beginning withzirconia-containing sols of Examples 7A through 7D respectively. Foreach sol, the sodium content was as noted above and the viscosity of theresin mixture was measured using the Bubble Time Method.

Table 6 shows the refractive index for Examples 8A to 8E. The refractiveindex increased from 1.535 for the unfilled resin (i.e., no zirconia) toapproximately 1.645 for the filled resins (i.e., containing zirconia).Table 6 also shows that the viscosity of the ZrO₂ resin dispersion isdependant on the amount of sodium cation in the feedstock. That is, theviscosity increased with the concentration of sodium cation.

TABLE 6 Effect of Feedstock Sodium Content on Viscosity of ZrO₂ resindispersion milligrams sodium ÷ Refractive Bubble Relative Example gramszirconium Index time Viscosity Ex 8A 0.276 1.6455 6.54 1 Ex 8B 0.5831.6450 8.62 1.318 Ex 8C 0.889 1.6462 11.86 1.813 Ex 8D 1.503 1.644834.61 5.292 Ex 8E 2.116 1.6430 85 12.997

Example 9A to 9D

Zirconia-containing sols were made with different levels of calcium. Thefeedstock used in Example 5 was spiked with various amounts of CaCl₂.The CaCl₂ was dissolved with DI water to prepare a solution with 2.5weight percent solids. The amounts of added 2.5 weight percent CaCl₂solution can be found in Table 7.

The reactions in the hydrothermal reactor were run with a feedstock thatcontained 15 weight percent solids (6.66 weight percent Zr, 0.295 weightpercent Y). The zirconia-containing sols were made using a proceduresimilar to Example 5, with the charges given in Table 7 below, at atemperature of 207° C. and using a 35 min residence time in HydrothermalReactor B. The calcium content of Example 9A was measured by ICP but wasbelow the detection limit of 0.052 milligrams calcium per gram ofzirconium. This amount is reported as zero in Table 7. The calciumcontent of each Examples 9B to 9D was calculated based on the amountthat was added.

TABLE 7 Reaction Conditions for Runs at Different Levels of CalciumZirconium Weight of milligrams Acetate Yttrium DI CaCl₂ solution calcium÷ Solution Acetate water added (2.5 grams Example (grams) (grams)(grams) wt-%) (grams) zirconium Ex 9A 900 24.2 1294 0 0 Ex 9B 900 24.21294 6.61 0.306 Ex 9C 900 24.2 1294 13.23 0.613 Ex 9D 900 24.2 1294 39.71.840

The resulting sols showed larger particle size with increasing amountsof calcium cation added to the feedstock before heating (Table 8). Theviscosity and the clarity of the aqueous sol are also affected by theaddition of calcium cation to the feedstock. Lower calcium levels leadto lower aqueous viscosity and higher clarity.

TABLE 8 Particle Characterization milligrams Poly- Volume calcium ÷Z-Average dispersity Average Example grams zirconium Size (nm) IndexSize (nm) Ex 9A 0 13.7 0.347 5.73 Ex 9B 0.306 13.9 0.332 6.39 Ex 9C0.613 15.2 0.273 7.84 Ex 9D 1.840 20.3 0.260 11.3

Example 10A to 10D

The zirconia-containing sols of Examples 9A to 9D were concentrated to40.5 weight percent zirconia via rotary evaporation (28.78 weightpercent Zr, 1.27 weight percent Y). The zirconia particles were surfacetreated and incorporated into a curable resin at 53 weight percent ZrO2as described for Examples 10A to 10D below.

Example 10A was prepared from Example 5. The zirconia-containing solconcentrated from Example 5 (50.01 grams at 41.89 weight percent ZrO2),MEEAA (2.46 grams), and 1-methoxy-2-propanol (35 grams) were charged toa 500 mL round-bottom flask in that order. HEAP from Preparative Example1 (4.26 grams at 50 weight percent in 1-methoxy-2-propanol), maleic acid(0.067 grams), and 1-methoxy-2-propanol (6.01 grams) were mixed togetherand heated to dissolve the maleic acid. This solution was then chargedto the above flask. PROSTABB 5198 (0.29 grams at 5 weight percent in DIwater) and Resin 1 (13.98 grams with a refractive index of 1.535) werethen charged to the flask. The volatiles were removed via rotaryevaporation to yield a curable acrylate resin mixture containingapproximately 53 weight percent ZrO₂.

Samples 10B through 10D were made in the same manner beginning with thezirconia-containing sols of Examples 9B through 9D respectively. Theviscosity of the resin mixture was measured using the Bubble TimeMethod.

Table 9 shows that the refractive index for all the samples increasedfrom 1.535 for the unfilled resin to approximately 1.645 for the filledsystems. Table 9 also shows that the viscosity of the ZrO₂ resindispersion was dependant on the amount of calcium cation in thefeedstock.

TABLE 9 Effect of Feedstock Calcium Content on Viscosity of ZrO₂ resindispersion milligrams calcium ÷ Bubble Relative Refractive Example gramszirconium time Viscosity Index Ex 10A 0 6.54 1 1.6455 Ex 10B 0.306 8.151.246 1.6456 Ex 10C 0.613 11.9 1.819 1.6450 Ex 10D 1.840 166 25.381.6448

Example 11A to 11C and 11A-IER to 11C-IER

Example 11A was prepared without ion exchange treatment using zirconiumacetate solution B. DI water (1213 grams) was charged to zirconiumacetate solution B (900 grams) to give a 15 weight percent solidsfeedstock. Yttrium acetate (24.03 grams) was added and the mixturestirred for approximately 12 hours. The solution was pumped at a rate of13.77 mL/min through Hydrothermal Reactor B. The tubular reactor wasimmersed in a bath of oil heated to 206° C. A clear, crystalline-ZrO2sol was produced.

Example 11A-IER was prepared with ion exchange treatment using zirconiumacetate solution B. DI water (1000 grams) was charged to zirconiumacetate solution B (900 grams). Ion exchange resin (AMBERLITE IR120 inhydrogen form) (25 grams) was added. This mixture was stirred overnight. The Ion exchange resin was removed via filtration. Yttriumacetate (24.21 grams) and DI water (285 grams) was added and the mixturestirred for about 12 hours. The solution was pumped at a rate of 13.77mL/min through Hydrothermal Reactor B. The tubular reactor was immersedin a bath of oil heated to 206° C. A clear, crystalline-ZrO₂ solresulted.

Similar procedures were used for Examples 11B and 11B-IER starting withzirconium acetate solution C and for Examples 11C and 11C-IER startingwith zirconium acetate solution A. The data are given in Table 10 below.ICP was used to determine the amount of sodium and calcium in eachfeedstock. Table 10 shows that the ion exchange resin removes the sodiumand calcium ions. It also shows that the particle size of the resultantsol is diminished when ion exchange is used to prepare feedstocks fromzirconium acetate solutions B and C. It should be noted that zirconiumsolution A did not have a high amount of impurities to start with andtherefore not much of a difference is expected.

TABLE 10 Effect of calcium and sodium on size milligrams milligramssodium ÷ calcium ÷ Volume ZrAc grams grams Z-Average PolydispersityAverage Example Solution zirconium zirconium Size (mm) Index Size (mm)Ex 11A B 2.106 <0.052 23.9 0.266 13.2 Ex 11A-IER B <0.023 <0.052 13.70.283 6.12 Ex 11B C 0.973 0.267 21.7 0.281 11.1 Ex 11B-IER C <0.023<0.052 13.8 0.373 5.45 Ex 11C A 0.259 <0.052 13.7 0.347 5.73 Ex 11C-IERA <0.023 <0.052

Examples 12A to 12C and 12A-IER to 12C-IER

The zirconia-containing sols of Example 11A, 11A-IER, 11B, 11B-IER, 11C,and 11C-IER were concentrated to 40.5 weight percent zirconia via rotaryevaporation. The zirconia particles were surface treated andincorporated into a curable resin at 53 weight percent ZrO2 as describedbelow.

Example 12A was prepared from Example 11A. The ZrO₂ sol concentratedfrom Example 11A (50.06 grams at 42.14 weight percent ZrO₂), MEEAA (2.48grams), and 1-methoxy-2-propanol (35 grams) were charged to a 500 mLround bottom flask in that order. HEAP from Preparative Example 1 (4.29grams at 50 weight percent in 1-methoxy-2-propanol), maleic acid (0.067grams), and 1-methoxy-2-propanol (6.01 grams) were mixed together andheated to dissolve the maleic acid. This solution was then charged tothe above flask. PROSTABB 5198 (0.30 grams at 5 weight percent in DIwater) and Resin 1 (14.01 grams) were then charged to the flask. Thevolatiles were removed via rotary evaporation to yield a filled curableacrylate resin mixture that contained approximately 53 weight percentZrO₂.

Examples 12A-IER, 12B, 12B-IER, 12C, and 12C-IER were prepared using anidentical procedure from Examples 11A-IER, 11B, 11B-IER, 11C, and11C-IER, respectively.

Table 11 shows that the refractive index for all the resin dispersionsincreases from 1.535 for the unfilled resin to approximately 1.645 forthe filled systems. The viscosity was measured via the bubble timemethod described above. The viscosity of the ZrO₂ resin dispersion canbe lowered significantly by removal of sodium and calcium cations fromthe feedstock by means of an ion exchange resin before making the sol.

TABLE 11 Effect of sodium and calcium on relative viscosity andrefractive index ZrAc Bubble Relative Refractive Example Solution timeViscosity Index Ex 12A B 74 12 1.6365 Ex 12A -IER B 6.61 1 1.6455 Ex 12BC 105 16 1.6444 Ex 12B -IER C 7.73 1 1.644 Ex 12C A 6.54 1 1.6455 Ex 12C-IER A 6.41 1 1.6451

Examples 13A to 13C

Example 13A was prepared as described in Example 5. Sodium chloride wasadded to portions of Example 13A to prepare Example 13B and sodiumnitrate was added to portions of Example 13A to prepare Example 13C. Theparticle size was measured using Photon Correlation Spectroscopy. Asnoted in Table 12, the presence of sodium ions does not affect theparticle size of the sol.

TABLE 12 Effect of sodium addition to zirconia sol on the particle sizemilligrams Volume sodium ÷ Z-Average Average Example grams zirconiumSize (nm) Size (nm) Ex 13A 15.5 5.08 Ex 13B 1.849 15.6 6.27 Ex 13C 1.84915.7 6.13

Examples 14A and 14B

Example 13A was concentrated to 41.04 weight percent zirconia via rotaryevaporation to provide Example 14A. Sodium chloride was added to oneportion of this sol to provide Example 14B. The zirconia particles inboth portions of the sol were surface treated and incorporated into acurable resin at 53 weight percent ZrO₂.

Example 14A was prepared from Example 13A without the addition of moresodium chloride. The zirconia-containing sol of Example 13A (50.01 gramsat 41.04 weight percent ZrO₂), MEEAA (2.42 grams), and1-methoxy-2-propanol (35.07 grams) were charged to a 500 mL round bottomflask in that order. HEAP from Preparatory Example 1 (4.18 grams at 50weight percent in 1-methoxy-2-propanol), maleic acid (0.065 grams), and1-methoxy-2-propanol (6.07 grams) were mixed together and heated todissolve the maleic acid. This mixture was then charged to the aboveflask. PROSTABB 5198 (0.29 grams at 5 weight percent in DI water) andResin 1 (13.64 gram) were then charged to the flask. The volatiles wereremoved via rotary evaporation to yield a filled curable acrylate resinmixture containing approximately 53 weight percent ZrO₂.

Example 14B was prepared from Example 13A with the addition of moresodium chloride. The zirconia-containing sol of Example 13A (200 gramsat 41.04 weight percent ZrO₂) and NaCl (11.48 grams of a 2.5 weightpercent solution) were mixed together. This spiked zirconia-containingsol (50.01 g at 38.81 weight percent ZrO₂), MEEAA (2.29 grams) and1-methoxy-2-propanol (35.4 grams) were charged to a 500 mL RB flask inthat order. HEAP from Preparatory Example 1 (3.95 grams at 50 weightpercent in 1-methoxy-2-propanol), maleic Acid (0.0618 grams), and1-methoxy-2-propanol (6.0 grams) were mixed together and heated todissolve the maleic acid. This mixture was then charged to the aboveflask. PROSTABB 5198 (0.28 grams at 5 weight percent in DI water) andResin 1 (12.90) were then charged to the flask. The volatiles wereremoved via rotary evaporation to yield a filled curable acrylate resinmixture containing approximately 53 weight percent ZrO₂.

The viscosities of Examples 14A and 14B were measured using the bubbletime method. The results in Table 13 show that addition of sodium ion toa finished sol does not affect the resin viscosity.

TABLE 13 Effect of sodium addition to zirconia sol on the relativeviscosity and refractive index of composite material milligrams sodium ÷Bubble Relative Refractive Example grams zirconium time Viscosity IndexEx 14A 5.8 1 1.6455 Ex 14B 1.849 6.26 1 1.64602

Examples 15A to 15B and Comparative Examples 15A to 15D

Six feedstock solutions were prepared using three different zirconiumsources. Example 15A was prepared by diluting zirconium acetate solutionA (16.3 weight percent zirconium) with DI water to prepare a feedstockthat contained 5.32 weight percent Zr. Yttrium acetate was added in anamount so that the final zirconia would contain about 4 weight percentyttrium oxide.

Example 15B was prepared by diluting zirconium acetate solution A (16.3weight percent zirconium) with DI water to prepare a feedstock thatcontained 1.10 weight percent Zr. Yttrium acetate was added in an amountso that the final zirconia would contain about 4 weight percent yttriumoxide.

Comparative Example 15A was prepared by diluting a solution of zirconiumdichloride oxide octahydrate (25 weight percent zirconium) and yttriumacetate with DI water to prepare a feedstock that contained 5.32 weightpercent Zr. Yttrium acetate was added in an amount so that the finalzirconia would contain about 4 weight percent yttrium oxide. ComparativeExample 15B was prepared by diluting a solution of zirconium dichlorideoxide octahydrate (25 weight percent zirconium) and yttrium acetate withDI water to prepare a feedstock that contained 1.10 weight percent Zr.Yttrium acetate was added in an amount so that the final zirconia wouldcontain about 4 weight percent yttrium oxide.

Comparative Example 15C was prepared by diluting a solution of zirconiumoxynitrate (31.78 weight percent zirconium) and yttrium acetate with DIwater to prepare a feedstock that contained 5.32 weight percent Zr.Yttrium acetate was added in an amount so that the final zirconia wouldcontain about 4 weight percent yttrium oxide.

Comparative Example 15D was prepared by diluting a solution of zirconiumoxynitrate (31.78 weight percent zirconium) and yttrium acetate with DIwater to prepare a feedstock that contained 1.10 weight percent Zr.Yttrium acetate was added in an amount so that the final zirconia wouldcontain about 4 weight percent yttrium oxide.

Each of the feedstocks was placed into a general purpose acid digestionbomb with a Teflon cup (Parr Model number 4749). The acid digestionbombs were placed in a forced air oven at a temperature of 200° C. for 4hr. They were then cooled and opened. Only the high concentrationacetate sol (Example 15) resulted in a clear sol. The chloride andnitrate sols yield milky materials and yielded no tetragonal phase.

The compositions of these examples are summarized in Table 14 and theXRD data is summarized in Table 15.

TABLE 14 Summary of Example 15-16 and Comparative Examples 15A-15D AddedYttrium Weight Counter Feedstock Acetate DI Water percent Example ion(grams) (grams) (grams) Zr Ex 15A Acetate 32.09 g 0.86 67.05 5.32 Ex 1BAcetate 6.68 0.179 93.14 1.10 CEx 15A Chloride 21.21 0.86 77.93 5.32 CEx15B Chloride 4.42 0.179 95.401 1.10 CEx 15C Nitrate 16.76 0.86 82.385.32 CEx 15D Nitrate 3.49 0.179 96.331 1.10

TABLE 15 Effect of the anion of the zirconium salt M M size C/T C/T sizeExample Appearance Intensity (nm) intensity (nm) Ex 15A Clear sol 234.0/7.5 100 12.5 Ex 15B Cloudy sol CEx 15A Milky sol 100 3.0/4.5 0 CEx15B Milky sol CEx 15C Milky sol 100  6.0/10.5 0 CEx 15D Milky sol

Example 16A to 16K and Comparative Examples 16A to 16C

The percent conversion of the feedstock was calculated based onthermogravimetric analysis as described above. The value of A, whichcorresponds to the percent weight loss of the feedstock, was 52.71percent with a standard deviation of 1.12. The value of A is an averageof 8 lots of zirconium acetate. The value of C, which corresponds to thepercent weight loss of a zirconia-containing standard believed to becompletely converted, was 8.71 percent. The value of B, whichcorresponds to the percent weight loss of the sample under analysis, isshown in Table 16 along with the calculation of the percent conversionfor various examples and comparative examples described above. All ofthe examples and comparative examples are nearly 100 percent convertedto zirconia-containing material.

TABLE 16 Percent conversion of feedstock to zirconia-containingnanoparticles. ZrO2 Residue at Residue at Percent Example Sample 200° C.900° C. B conversion Ex 16A Ex 1 97.99 90.14 8.71 100 Ex 16B Ex 2 97.9290.38 8.34 100.83 Ex 16C Ex 3 97.98 90.97 7.70 102.27 Ex 16D Ex 4 97.7589.97 8.65 100.14 Ex 16E Ex 6 98.43 89.99 9.37 98.48 Ex 16F Ex 7A 98.3690.65 8.51 100.46 Ex 16G Ex 9B 98.01 90.31 8.53 100.4 Ex 16H Ex 9D 98.0290.35 8.49 100.50 Ex 16I Ex 11A 97.77 89.88 8.78 99.84 Ex 16J Ex 11A-IER97.79 89.71 9.01 99.32 Ex 16K Ex 11B-IER 97.95 89.89 8.97 99.41 CEx 16ACEx 1 98.03 90.93 7.808 102.04 CEx 16B CEx 2 97.89 91.23 7.30 103.2 CEx16C CEx 3 96.99 88.31 9.83 97.45

Example 17

DI water (1929 grams) was mixed with zirconium acetate (solution A, 2100grams). Yttrium acetate (56.49 grams) was added and the mixture wasstirred for approximately 12 hours to prepare the 19 weight percentsolids feedstock (8.44 weight percent Zr, 0.374 weight percent Y). Thefeedstock was pumped at a rate of 11.5 mL/min through HydrothermalReactor B. A clear, crystalline-ZrO₂ containing sol resulted.

The resultant sol was dialyzed against DI water for approximately 20hours using a SPECTROPOR Membrane (MWCO 12-14,000), available fromSpectrum Labs (Rancho Domoinqeus Calif.). The water was changed numeroustimes. This step removed excess acetic acid from the sol. The resultingsol was concentrated to 32.74 weight percent ZrO2 via rotaryevaporation.

The concentrated sol (200 grams) was charged to a 16 ounce jar. SILQUESTA1230 (9.62 grams), 1-methoxy-2-propanol (225 grams), methacryloxypropyltrimethoxysilane (14.38 grams) and PROSTABB 5198 (0.25 grams of 5 weightpercent solids in DI water) were added to sol in that order. The mixturewas mixed and then the jar sealed and heated to 90° C. for 4 hours. Theresultant mixture (446.53 grams) was charged to a 1000 ml round bottomflask and concentrated to 223.26 grams.

A 2000 mL beaker was charged with DI water (450 grams) and concentratedammonia (7.05 g). The above surface modified ZrO2 sol was poured intothe aqueous ammonia and stirred slowly for 10 minutes. This resulted ina white precipitate. The precipitate was isolated on a Buchner funnel(Whatman #4 filter paper). It was washed twice with 100 mL of DI water.The resultant damp solids (245 grams) were then dissolved in1-methoxy-2-propanol (253 g). This mixture (453.6 grams) wasconcentrated to 207.6 grams via rotary evaporation. This yields a sol of66.8 weight percent surface treated ZrO2 particles.

A 100 mL round bottom flask was charged with the concentrated ZrO2 sol(50 grams at 66.8 weight percent solids), hydroxyethyl methacrylate(15.6 grams) and SR 603 (6.7 grams). The solvent was removed via rotaryevaporation and the sample was concentrated to 55.4 grams. The resultingsample was a translucent, low viscosity sol of silane treated ZrO2 in acurable methacrylate resin. The refractive index of 1.564 isconsiderably higher than of the unfilled resin (1.4555).

What is claimed is:
 1. A continuous hydrothermal reactor systemcomprising: a tubular reactor comprising a composite tube comprising (a)a metal sheath and (b) a fluorinated polymeric hose encased within themetal sheath, wherein the tubular reactor is coiled; a pump connected toa first end of the tubular reactor, wherein the pump continuouslyintroduces a feedstock comprising an aqueous medium into the first endof the tubular reactor; a means of heating and controlling thetemperature of the feedstock within the tubular reactor; a coolingdevice connected to a second end of the tubular reactor that is oppositethe first end, wherein an effluent is continuously removed from thetubular reactor and passed through the cooling device; and a means ofcontrolling pressure within the tubular reactor, said means ofcontrolling pressure comprising a backpressure valve connected to anexit of the cooling device, wherein pressure and temperature areselected to subject the feedstock to hydrothermal treatment conditionswithin the tubular reactor.
 2. The continuous hydrothermal reactorsystem of claim 1, wherein the fluorinated polymeric hose comprises afluorinated polyolefin.
 3. The continuous hydrothermal reactor system ofclaim 1, wherein the fluorinated polymeric hose comprisespolytetrafluoroethylene.
 4. The continuous hydrothermal reactor systemof claim 1, wherein the tubular reactor has an inner diameter in a rangeof 0.1 to 3.0 centimeters.
 5. The continuous hydrothermal reactor systemof claim 1, wherein the means of heating comprises a heating mediumvessel containing a heating medium comprising an oil, sand, or salt, andwherein the tubular reactor is positioned in the heating medium.
 6. Thecontinuous hydrothermal reactor system of claim 5, wherein the heatingmedium is an oil.
 7. The continuous hydrothermal reactor system of claim5, wherein the heating medium heats the tubular reactor to a reactortemperature in a range of 170° C. to 275° C.
 8. The continuoushydrothermal reactor of claim 1, wherein the pressure at the exit of thehydrothermal reactor is at least 200 pounds per square inch and thetemperature within the tubular reactor is at least 170° C.
 9. Acontinuous hydrothermal reactor system comprising: a tubular reactorcomprising a composite tube comprising (a) a metal sheath that isbraided and (b) a fluorinated polymeric hose encased within the metalsheath; a pump connected to a first end of the tubular reactor, whereinthe pump continuously introduces a feedstock comprising an aqueousmedium into the first end of the tubular reactor; a means of heating andcontrolling the temperature of the feedstock within the tubular reactor;a cooling device connected to a second end of the tubular reactor thatis opposite the first end, wherein an effluent is continuously removedfrom the tubular reactor and passed through the cooling device; and ameans of controlling pressure within the tubular reactor, said means ofcontrolling pressure comprising a backpressure valve connected to anexit of the cooling device, wherein pressure and temperature areselected to subject the feedstock to hydrothermal treatment conditionswithin the tubular reactor.
 10. The continuous hydrothermal reactorsystem of claim 9, wherein the metal sheath comprises stainless steel.11. The continuous hydrothermal reactor system of claim 9, wherein thefluorinated polymeric hose comprises a fluorinated polyolefin.
 12. Thecontinuous hydrothermal reactor system of claim 9, wherein thefluorinated polymeric hose comprises polytetrafluoroethylene.
 13. Thecontinuous hydrothermal reactor system of claim 9, wherein the tubularreactor has an inner diameter in a range of 0.1 to 3.0 centimeters. 14.The continuous hydrothermal reactor system of claim 9, wherein the meansof heating comprises a heating medium vessel containing a heating mediumcomprising an oil, sand, or salt, and wherein the tubular reactor ispositioned in the heating medium.
 15. The continuous hydrothermalreactor system of claim 14, wherein the heating medium is an oil. 16.The continuous hydrothermal reactor system of claim 14, wherein theheating medium heats the tubular reactor to a reactor temperature in arange of 170° C. to 275° C.
 17. The continuous hydrothermal reactor ofclaim 9, wherein the tubular reactor is coiled.
 18. The continuoushydrothermal reactor of claim 9, wherein the pressure at the exit of thehydrothermal reactor is at least 200 pounds per square inch and thetemperature within the tubular reactor is at least 170° C.